Company letter numbering formats: components, examples by division and SOP rules

Letter numbering looks trivial until someone has to find a letter, audit it or prove when it was issued. This guide covers the components of a company letter number and how to order them, with examples for each division. It ends with the rules worth writing into your SOP so that numbers stay clean when several people number letters at once.

Private companies in Indonesia have no single letter numbering format required by law. The formats below are common practice, so you can adapt them as long as your rules are consistent and written down. Government agencies follow their own official correspondence guidelines, which this guide does not cover.

Why letter numbering matters

A letter number is the key to finding a letter again. Three reasons stand out.

• Tracing. When a business partner writes "referring to your letter number 0142", your team should be able to find that letter in minutes.
• Archiving. A numbering pattern makes archive folders and registers easy to organise, and letters from one division or one year are easy to group.
• Audit. Auditors usually check whether the number sequence runs without breaks. A number that jumps without a recorded reason raises questions about missing or deleted letters.

Common components

Company letter numbers are usually built from the following components.

You do not need every component. A small company is often fine with a sequence number, a division code and a year, while a company with heavy correspondence usually uses all five.

How to order them

The most common order puts the sequence number first, followed by the letter-type code, the division code, the Roman-numeral month and the year, separated by slashes:

042/SK/HRD/IV/2026

The sequence number goes first because it is the part people search and sort by most often. Some companies put the division code first so that a per-division register reads easily, and that is also fine. What matters is that everyone uses the same order and that you do not change it in the middle of a year.

A few habits help:

• Fix the number of digits in the sequence, for example three or four (042 or 0042), so that the register sorts correctly.
• Use one separator, such as the slash, and do not mix it with hyphens.
• Avoid abbreviations that only one person understands. The list of codes belongs in the SOP.

Examples by division

This table is only an illustration. Adapt the letter-type and division codes to your own structure.

Incoming and outgoing letters, and per unit

You issue outgoing letters, so you assign their numbers. Incoming letters arrive with the sender's own number, which you only record as a reference. Your company still needs its own number for each incoming letter, usually called an agenda number, so that every letter received has an internal identity.

For that reason, keep two separate series:

• Outgoing series, for example 042/SK/HRD/IV/2026.
• Incoming agenda series, for example AGD/0213/2026.

For separation by unit, there are two common choices. The first is one company-wide sequence with the division code as a marker. The second is a sequence that runs separately in each division, so HR and Finance both have a 001 at the start of the year. The second is easier to manage when each division has its own administrator, while the first is easier to audit. Whichever you choose, write it in the SOP and do not mix the two.

Yearly reset

Most companies restart the sequence at 001 on 1 January. Because the year is part of the number, 001/.../2025 and 001/.../2026 never clash. Companies with very high letter volume sometimes reset monthly, which also works as long as the month is in the number.

Write the reset date into the SOP. A letter signed on 30 December but issued on 2 January needs a clear answer on which year's series it belongs to. The simplest rule is that the year follows the date on which the number is issued.

Voided numbers, gaps and backdated letters

This is where most problems appear, so the rules need to be firm.

Voided numbers

If a letter is cancelled after its number was taken, do not delete the row and do not reuse the number. Mark it as void and record the date, who voided it and why. An auditor can then see that the gap has an explanation.

Gaps

A gap caused by a typing error or a cancelled letter should be recorded in the register as "not used", with a note. Do not close the gap by shifting the next letter's number, because a number that has already been communicated outside the company must not change.

Backdated letters

Avoid backdating letters. If you truly cannot, for example because a letter was stuck in processing, give it the next number in the real order of issue, write the real date in the register and keep a record of your manager's approval. Forcing an old number onto a new letter means either reusing a number that is already taken or slipping in a number that should have passed. Both damage the integrity of the register.

Collisions when two people number at once

The classic problem with a shared spreadsheet is two people reading the same last row and both typing the next number. To prevent it:

1. Name one person or one role as the issuer of numbers for each series.
2. Take the number first and write the letter afterwards. Do not guess the "probable next number" in a draft.
3. If two identical numbers have already gone out, do not change either one quietly. Record the incident, decide which letter keeps the number, and issue a new number for the other letter with a correction note.

Writing it into your SOP

A good numbering SOP fits on one or two pages and covers the following.

1. The full number format with examples.
2. The list of letter-type and division codes, and who may add a new code.
3. The series rules: outgoing and incoming agenda kept apart, and whether sequences are separate per unit.
4. The reset rule (yearly or monthly) and the reference date.
5. Who may issue numbers and how a number is requested.
6. The procedure for voided numbers, gaps and backdated letters.
7. Where the register lives and who checks it for completeness, for example once a month.

Common mistakes

• Keeping several register files that drift out of sync, so the same number is issued twice.
• Changing the format mid-year without recording when the change took effect.
• Deleting the rows of cancelled letters, which leaves a sequence that looks complete but has lost its history.
• Mixing incoming and outgoing letters in one number run.
• Letting each division invent its own codes without a master list.
• Retyping the number into the letter by hand, so the register and the letter can disagree.
• Never reviewing the register, so gaps are only discovered during an audit.

How Obscura helps

The Correspondence module in Obscura issues letter numbers under a configured scheme, for example 0014/STD/2026, and an administrator can apply a preset for an official format such as 042/SK/IV/2026. Each scheme has one sequence, so separate number runs need separate schemes, and a new sequence can start yearly, monthly or daily as configured.

Every number in the register is recorded as reserved, assigned or voided, and none is silently skipped or reused. Duplicates are prevented within the same scheme and period, an issued number cannot be changed, and issued number records are protected from deletion. Incoming letters are registered with an agenda number in the same register as outgoing letters. You can read the details on the Correspondence page.