If you received a subpoena from a Philippine prosecutor or you are preparing a counter-affidavit, the I.S. Number is one of the first details you should check. It is the prosecutor’s case reference number for the criminal complaint at the preliminary investigation or inquest stage. In practical terms, it helps the prosecutor’s office identify your file, match your counter-affidavit to the correct complaint, and track the case before it becomes, if ever, a court criminal case.

What Does I.S. Number Mean?

I.S. Number means Investigation Slip Number. Older DOJ and prosecution documents use “I.S. No.” to refer to the docket or reference number assigned to a complaint filed with the Office of the City Prosecutor, Office of the Provincial Prosecutor, Regional Prosecution Office, or the Department of Justice.

The term appears in DOJ appeal rules. For example, the 2000 NPS Rule on Appeal required a petition for review to state the “Investigation Slip number (I.S. No.)” and the criminal case number, if any. (Supreme Court E-Library)

Today, many prosecution offices use the term NPS Docket Number instead of I.S. Number. But in practice, you may still see any of these labels:

Term on the document	What it usually refers to
I.S. No.	Investigation Slip Number; older prosecutor docket number
NPS Docket No.	Current-style National Prosecution Service docket number
INV No.	Investigation docket number, usually for preliminary investigation
INQ No.	Inquest docket number, usually after a warrantless arrest
DOJ Docket No.	Docket number for a matter handled by the DOJ or appealed to the DOJ
Criminal Case No.	Court case number after an Information is filed in court

The important point is this: an I.S. Number is usually a prosecutor-level number, not a court case number.

Where the I.S. Number Appears in a Counter-Affidavit

A counter-affidavit is the respondent’s sworn written answer to a criminal complaint. It is usually filed after the respondent receives a subpoena from the prosecutor.

The I.S. Number usually appears in the caption, near the top of the first page, like this:

Republic of the Philippines
Department of Justice
National Prosecution Service
Office of the City Prosecutor
Quezon City

JUAN DELA CRUZ,
Complainant,

-versus-                                      I.S. No. XV-03-INV-26A-00123

PEDRO SANTOS,
Respondent.

For: Estafa under Article 315 of the Revised Penal Code

In this example:

XV-03 may refer to an office, region, or internal prosecution code.
INV usually suggests a preliminary investigation docket.
26 may refer to the year 2026.
A-00123 may be an internal month/series/serial number.
The exact format varies by city, province, region, and DOJ system.

Do not “correct” the format based on what you found online. Copy the I.S. Number exactly as written in the subpoena or complaint records, including letters, hyphens, zeros, and spacing.

Why the I.S. Number Matters

The I.S. Number is not just a technical detail. It affects how your counter-affidavit is received, routed, and matched to the correct case.

It is used to:

Identify the exact prosecutor case

Prosecutor offices handle thousands of complaints. Names can repeat. A docket number prevents confusion.
Match your counter-affidavit to the correct complaint

If the caption has the wrong I.S. Number, your filing may be misplaced, delayed, or questioned.
Request case status

When asking the docket or records section whether a resolution has been issued, the I.S. Number is usually the fastest reference.
File later pleadings

Motions, manifestations, reply-affidavits, rejoinders, motions for reconsideration, and petitions for review normally refer to the same docket number.
Trace whether the complaint became a court case

If the prosecutor finds sufficient basis to charge, an Information may be filed in court. At that point, the court assigns a separate Criminal Case No.

Does an I.S. Number Mean You Already Have a Criminal Case in Court?

Not necessarily.

An I.S. Number usually means that a criminal complaint has been filed with the prosecutor’s office and docketed for action. At that stage, you are usually called the respondent, not yet the accused in a court case.

A criminal case in court normally begins only after the prosecutor files an Information with the proper court, such as the Municipal Trial Court, Metropolitan Trial Court, Municipal Trial Court in Cities, Regional Trial Court, or other court with jurisdiction.

Here is the usual distinction:

Stage	Common case number	Who handles it	What it means
Barangay complaint	Barangay/Lupon number	Barangay Lupon	Conciliation stage for covered disputes
Police blotter	Blotter entry number	PNP station	Record of reported incident
Prosecutor complaint	I.S. No., NPS Docket No., INV No., or INQ No.	Prosecutor’s Office / DOJ-NPS	Complaint is under preliminary investigation or inquest
Court criminal case	Criminal Case No.	Court	Information has been filed in court

The National Prosecution Service is created under Republic Act No. 10071, the Prosecution Service Act of 2010, and is primarily responsible for preliminary investigation and prosecution of cases involving violations of penal laws under the supervision of the Secretary of Justice. (Lawphil)

Legal Basis: Counter-Affidavits and Preliminary Investigation

A counter-affidavit is part of the respondent’s opportunity to answer the accusations before the prosecutor decides whether a case should be filed in court.

Historically, Rule 112 of the Rules of Criminal Procedure governed preliminary investigation. In 2024, the Supreme Court recognized the DOJ’s authority to issue its own DOJ-NPS rules on preliminary investigations and inquest proceedings, while acknowledging that preliminary investigation is an executive, not judicial, function. (Supreme Court E-Library)

The Supreme Court later upheld DOJ Department Circular No. 15, series of 2024, which raised the standard in preliminary investigations and inquests from probable cause to prima facie evidence with reasonable certainty of conviction. (Supreme Court of the Philippines)

In ordinary language, this means prosecutors are not supposed to file weak criminal cases in court just because a complaint was submitted. They must evaluate whether the available evidence is admissible, credible, capable of being preserved and presented, and sufficient to establish the elements of the offense if left uncontroverted.

How the I.S. Number Fits Into the Prosecutor Process

The exact flow may vary by office, but a typical criminal complaint before the prosecutor moves this way:

Complaint-affidavit is filed

The complainant files a sworn complaint-affidavit, witness affidavits, supporting documents, and an investigation data form.
The complaint is checked and docketed

If the filing is accepted, the office assigns a docket number. This may appear as an I.S. Number, NPS Docket Number, INV Number, or INQ Number.
The prosecutor reviews the records

The investigating prosecutor checks whether there is basis to continue.
A subpoena is issued

If the prosecutor proceeds, the respondent receives a subpoena with copies of the complaint-affidavit and supporting evidence.
The respondent files a counter-affidavit

The counter-affidavit should use the correct I.S. Number and case title.
The prosecutor may hold a hearing or require further affidavits

The prosecutor may ask clarificatory questions, require a reply-affidavit and rejoinder-affidavit, or submit the case for resolution.
The prosecutor issues a resolution

The resolution may recommend dismissal, filing of an Information in court, or other appropriate action.

Under current DOJ-NPS practice described in legal updates on the 2024 rules, the subpoena should include the complaint-affidavit and set a date for the respondent to submit a counter-affidavit, with at least 10 days from receipt. (DivinaLaw)

What to Check Before Filing a Counter-Affidavit

Before signing or filing your counter-affidavit, check the caption carefully.

1. Correct I.S. Number or NPS Docket Number

Use the number appearing in the subpoena or prosecutor records.

Common mistakes include:

confusing the letter “O” with zero “0”
missing hyphens
omitting the office code
using the police blotter number instead of the prosecutor docket number
using the barangay case number instead of the I.S. Number

2. Correct prosecutor’s office

The heading should match the office handling the case, such as:

Office of the City Prosecutor of Manila
Office of the City Prosecutor of Quezon City
Office of the Provincial Prosecutor of Cavite
Regional Prosecution Office
Department of Justice, National Prosecution Service

If the wrong office is written, the document may still identify the facts, but it can create avoidable confusion.

3. Correct party names

Check spelling, middle names, suffixes, business names, and aliases. Use the names as they appear in the subpoena, then clarify any errors in the body if necessary.

4. Correct offense

The caption usually says “For: Estafa,” “For: Qualified Theft,” “For: Cyberlibel,” “For: Violation of RA 9262,” or another offense.

The offense in the caption is not always final. The prosecutor may later find a different proper charge, downgrade the offense, or dismiss the complaint. Still, your counter-affidavit should answer the allegations actually served on you.

5. Correct deadline and filing mode

Do not rely only on a generic “10-day rule.” Read the subpoena or order. Under the 2024 DOJ-NPS rules, filings and service may involve physical filing, electronic filing in PDF, or submission of hard copies depending on the prosecutor’s order and available systems. Legal updates on the rules note that e-filing of counter-affidavits may be allowed when an extension is granted, but the original hard copy may still need to be submitted. (Global Litigation News)

Common Situations and What the I.S. Number Means

You received a subpoena with an I.S. Number

This usually means a complaint has been filed with the prosecutor and you are being required to answer. The subpoena should tell you:

the case title
the offense complained of
the prosecutor’s office
the investigating prosecutor, if already assigned
the date, time, and place of hearing or submission
the deadline for your counter-affidavit

Someone texted you an I.S. Number but you have no subpoena

A text message or screenshot is not enough to confirm the case. Verify with the correct prosecutor’s office. A real docket number should correspond to an actual office, parties, and case record.

Be careful with scams. Fake subpoenas sometimes use random docket numbers, old office logos, or threatening language demanding settlement money.

The document says “NPS Docket No.” instead of “I.S. No.”

That is usually normal. The more current label is often NPS Docket Number. Treat it as the prosecutor case reference number unless the office tells you otherwise.

The case already has a Criminal Case No.

If there is already a Criminal Case No., the prosecutor may have filed an Information in court. At that point, the matter may already be with a court branch. The prosecutor docket number remains useful for tracing the investigation history, but the court will usually track the case using the Criminal Case No.

You are abroad and need to sign a counter-affidavit

A counter-affidavit is sworn. If signed outside the Philippines, the receiving prosecutor’s office may require a consular notarization, apostille, or other acceptable authentication depending on where it was signed and how it will be used.

Philippine embassies and consulates can notarize private documents such as affidavits for use in the Philippines, usually requiring personal appearance. (Philippine Embassy) Some consulates also explain that affidavits and similar documents may be signed before a consular officer in the form of an acknowledgment or jurat. (Philippine Consulate General Toronto)

For practical filing, a respondent abroad should pay close attention to:

original signed copy
passport or valid ID copy
consular notarization or apostille, if required
courier time to the Philippines
whether the prosecutor allows initial e-mail filing
authorization for a Philippine representative to file or receive copies

Documents Usually Connected to an I.S. Number

Document	Who usually prepares it	Why it matters
Complaint-affidavit	Complainant	States the accusation under oath
Witness affidavits	Complainant or witnesses	Supports the complaint
Investigation Data Form	Complainant / prosecution office	Helps docket and classify the complaint
Subpoena	Prosecutor	Notifies respondent to answer
Counter-affidavit	Respondent	Respondent’s sworn defense
Supporting documents	Either party	Receipts, screenshots, contracts, IDs, photos, medical records, messages
Reply-affidavit	Complainant, if allowed or required	Responds to new matters in the counter-affidavit
Rejoinder-affidavit	Respondent, if allowed or required	Final response to the reply
Prosecutor’s resolution	Investigating prosecutor, approved by office head	Recommends dismissal or filing of charges
Information	Prosecutor	Formal charge filed in court

Mistakes to Avoid

Using the wrong number in the caption

If you put a police blotter number where the I.S. Number should be, the records section may still receive the document, but it creates risk. Always use the prosecutor docket number shown in the subpoena.

Filing a “position paper” instead of a sworn counter-affidavit

A counter-affidavit must be sworn. An unsworn explanation may not have the same value. It should normally include a jurat or proper oath portion.

Ignoring the attached complaint documents

The counter-affidavit should answer the specific complaint-affidavit and supporting evidence served on you. Do not merely deny everything in general terms.

Missing the deadline

If the respondent fails to file a counter-affidavit, the prosecutor may resolve the complaint based on the available records. Under the 2024 rules as summarized in legal updates, if the respondent fails to respond despite proper notice, the case may proceed to resolution. (DivinaLaw)

Assuming an I.S. Number means guilt

An I.S. Number does not mean the prosecutor has found you guilty. It means the matter has been recorded and is being processed. Guilt or innocence is determined by a court after trial, if the case reaches that stage.

Assuming settlement automatically cancels the I.S. Number

Even if parties settle, the prosecutor may still evaluate whether the offense involves public interest, whether the crime is one that can be compromised, and whether dismissal is legally proper. For example, private settlement does not automatically erase criminal liability for many offenses under the Revised Penal Code or special penal laws.

Frequently Asked Questions

What does I.S. No. mean in a counter-affidavit?

I.S. No. means Investigation Slip Number. It is the prosecutor’s docket or reference number for a criminal complaint at the investigation stage.

Is the I.S. Number the same as a criminal case number?

No. The I.S. Number usually belongs to the prosecutor’s office. A Criminal Case No. is assigned by the court after an Information is filed.

Does having an I.S. Number mean I will be arrested?

Not by itself. An I.S. Number only shows that a complaint has been docketed or processed at the prosecutor level. Arrest issues depend on separate rules, such as warrantless arrest, inquest, filing of Information, bail, and any warrant issued by a court.

What should I write if the subpoena says NPS Docket No. instead of I.S. No.?

Use the label and number shown in the subpoena. If it says NPS Docket No., write that. If it says I.S. No., write that. The important thing is accuracy.

Can I file a counter-affidavit without the I.S. Number?

It is risky. If you do not know the number, use the case title, complainant’s name, respondent’s name, offense, subpoena date, and prosecutor’s office, but the docket number should be verified as soon as possible.

Can the I.S. Number be used to check the status of the case?

Yes. The docket or records section of the prosecutor’s office commonly uses the I.S. Number or NPS Docket Number to locate the case record and check whether it is pending, submitted for resolution, dismissed, or filed in court.

What if the I.S. Number on my subpoena is different from the number on the complaint?

Do not guess which one is correct. The difference may be a typographical error, amended docketing, separate complaint, or inquest/preliminary investigation conversion. The safer approach is to copy both in your filing and state that you are referring to the subpoena and complaint served on you.

Is an I.S. Number used in civil cases?

Usually no. I.S. Numbers are associated with prosecutor-level criminal complaints. Civil cases use court docket numbers, while barangay matters, labor cases, immigration matters, and administrative cases have their own numbering systems.

Can a foreigner have an I.S. Number in a Philippine complaint?

Yes. Foreigners can be complainants or respondents in Philippine criminal complaints. If the foreigner is abroad, practical issues often involve notarization, apostille or consular acknowledgment, courier filing, and authority for a Philippine representative.

What happens after I file my counter-affidavit?

The prosecutor may submit the case for resolution, call a clarificatory hearing, require additional affidavits, or request more evidence. The final prosecutor resolution may recommend dismissal or the filing of an Information in court.

Key Takeaways

I.S. Number means Investigation Slip Number.
It is usually the prosecutor’s docket number, not the court’s Criminal Case No.
Many offices now use NPS Docket Number, but older documents and pleadings may still say I.S. No.
Always copy the number exactly from the subpoena or prosecutor record.
The I.S. Number helps the prosecutor’s office match your counter-affidavit to the correct case.
A counter-affidavit should be sworn, properly captioned, supported by evidence, and filed within the period stated in the subpoena or order.
If the case is later filed in court, it will receive a separate Criminal Case No.
For respondents abroad, notarization, apostille, consular acknowledgment, courier timing, and authorization for a representative can affect whether the filing is accepted on time.