> ## Documentation Index
> Fetch the complete documentation index at: https://docs.lead.bank/llms.txt
> Use this file to discover all available pages before exploring further.

# X9 File Requirements

> All check processing between you and Lead uses the X9.37 file standard.

The sections below cover the file structure and the fields Lead uses for routing, matching, and funds movements.

## File Structure

* Each X9 file contains one or more cash letters, each of which contains one or more bundles of check items.
* Each check item includes the front image, back image, and structured MICR data.
* Do not mix forwards and returns in the same X9 file.

## Key Fields

The following fields are critical for correct processing:

| **Field**                                | **Notes**                                                                                                                                                       |
| :--------------------------------------- | :-------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Settlement Date**                      | Lead uses this date to determine when to move funds between the Fed GL and the Checks In Process GL.                                                            |
| **Amount**                               | Integer value in cents. Lead uses this for daily limit checks and funds movements.                                                                              |
| **Routing Number**                       | Lead uses this to route incoming checks to the correct partner. For dedicated routing numbers, Lead forwards all items on the routing number to you.            |
| **OnUs Field (MICR)**                    | Contains the account number. Lead uses this for item matching on shared routing numbers. Lead works with each partner to configure the structure of this field. |
| **ECE Institution Item Sequence Number** | A mandatory 15-digit field. Lead uses this to match incoming returns to the original outgoing entry. Populate it consistently on all outgoing X9 files.         |
| **Return Reason**                        | Populated on return items. Indicates why a check is being returned — for example, an account closed or fraud detected.                                          |
| **Cash Letter ID**                       | An eight-character alphanumeric identifier for the cash letter within the file.                                                                                 |
| **BOFD Indicator**                       | Bank of First Deposit indicator. Mandatory field; valid values are Y, N, or U.                                                                                  |

## Return Matching

Lead matches incoming returns to original outgoing entries using the **ECE Institution Item Sequence Number** and the submission date. Populate this field consistently on all outgoing X9 files to enable accurate return matching.