---
title: "ACH, BAI2 and X9.37 File Reference | NACHA Navigator"
description: "Record layouts, code lists and validation rules for NACHA (ACH) files, BAI2 balance reports and X9.37 image cash letters, plus sample files."
canonical: https://nacha-navigator.com/reference
updated: 2026-10-30
---

# Payment file reference

Reference

Record layouts, code lists and validation rules for the files NACHA Navigator reads: NACHA (ACH) files, BAI2 balance reports and X9.37 image cash letters.

Last updated October 30, 2026

The layouts and code lists on these pages come from the same tables the viewers use to read and check a file, so the pages and the tools agree. When you have a file in hand, open it in a viewer to see the same fields labeled with your own data.

## NACHA (ACH) files

The fixed-width files banks exchange to move ACH payments: payroll, vendor and bill payments, and the returns that come back.

Open a file in the [NACHA Viewer](https://nacha-navigator.com/), or build one with the [NACHA Generator](https://nacha-navigator.com/nacha-generator).

### [NACHA file format](https://nacha-navigator.com/nacha-file-format)

What every field of every record holds: its position, length and format, whether it is mandatory, and the codes that go in it.

### [NACHA file errors](https://nacha-navigator.com/nacha-file-errors)

Why a file gets rejected: each error and warning the NACHA Viewer reports, what it means and how to fix it.

### [ACH transaction codes](https://nacha-navigator.com/ach-transaction-codes)

What the two digits in positions 2–3 of an entry mean: the account, credit or debit, and whether it is a live entry, a prenote, a zero-dollar entry or a return.

### [ACH reversal](https://nacha-navigator.com/ach-reversal)

When an entry or a whole file may be reversed, and how the reversing file is built from the original. Build one in your browser.

## BAI2 balance reports

The balance and transaction reports banks send their business customers, account by account.

Open a file in the [BAI2 Viewer](https://nacha-navigator.com/bai2-viewer).

### [BAI2 type codes](https://nacha-navigator.com/bai2-type-codes)

What a three-digit type code means: its description, whether it reports a balance, a total or a single transaction, and whether it is a credit or a debit.

### [BAI2 file format](https://nacha-navigator.com/bai2-file-format)

Every record from the 01 file header to the 99 file trailer, field by field, with funds types, continuation records and control totals.

## X9.37 image cash letters

The files of check images and MICR data that banks exchange to clear checks electronically.

Open a file in the [X9 Viewer](https://nacha-navigator.com/x9-viewer).

### [X9.37 file format](https://nacha-navigator.com/x9-file-format)

Every record type of an image cash letter, from the file header to the check images, with its fields.

## Sample files

Made-up data in real formats, for learning a format or testing your own software.

### [Sample files](https://nacha-navigator.com/sample-files)

Sample NACHA, BAI2 and X9.37 files to download or open in the viewers, with what each one shows.

## Related free tools

Code lookups and ISO 20022 files have their own free sites, from the same maker as NACHA Navigator.

### [Payment code lookup](https://payopskit.com/codes)

Look up ACH return reason, notification of change and SEC codes on PayOps Kit.

### [ISO 20022 Navigator](https://iso20022-navigator.com/)

Viewers for ISO 20022 camt.053 statements and pain.001 payment files, and for MT940.

Missing a reference you need, or found a mistake? [Email us](mailto:davidkaylor95+nachanav@gmail.com).