> ## 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.

# Payment Identifiers

> Reference end_to_end_id, transaction_id, and uetr identifiers used to track FedNow and RTP instant payments and returns across the Lead network.

The `instant_payment` object has an `id` of the form `instant_payment_xyz123` that references it internally within the Lead network. Additionally, Lead exposes three identifiers that identify the payment message within the payment network. None of these identifiers are configurable through the `instant_payments` API. Instead, Lead sets them automatically for outgoing payments and the counterparty sets them for incoming payments.

1. `end_to_end_id` is the mandatory "trace id" for instant payments that must pass through from debtor to creditor unchanged. Use it to reference a payment with the end customer or a counterparty. `end_to_end_id` is a max of 35 characters and is expected to be unique.
2. `transaction_id` is an optional identifier assigned by the debtor or initiating party that may also be used to reference a specific payment on the network. `transaction_id` is a max of 35 characters and is expected to be unique.
3. `uetr` is an optional identifier assigned by the debtor or initiating party that may also be used to reference a specific payment on the network. `uetr` is a UUIDv4 that is expected to be globally unique.

## Payments

Payments will include two or three payment identifiers which depend on the underlying rail and the party initiating the payment. Outgoing payments from Lead will always have `end_to_end_id` and `transaction_id` populated. FedNow and RTP require usage of `end_to_end_id` because it is the primary identifier to track a payment with a counterparty or an end customer. FedNow also mandates use of either `transaction_id` or `uetr`, but RTP mandates `transaction_id` and leaves `uetr` as optional.

| Instant Payment ID | Required by FedNow?             | Required by RTP? | Network Message ID | Validation        |
| ------------------ | ------------------------------- | ---------------- | ------------------ | ----------------- |
| `end_to_end_id`    | Yes                             | Yes              | `EndToEndId`       | Max 35 characters |
| `transaction_id`   | Yes if `uetr` is null           | Yes              | `TxId`             | Max 35 characters |
| `uetr`             | Yes if `transaction_id` is null | No               | `UETR`             | UUID V4           |

## Returns

Payments and returns both utilize their own `instant_payment` object. Returns are identified with a non-null `instant_payment.related_objects.original_payment_id`. FedNow returns do not have any of their own `instant_payment.payment_identifiers`. To reference a FedNow return, use the `end_to_end_id` of the original payment. RTP returns, however, support the same set of identifiers that the original payment supports. To reference an RTP return, simply look at the `end_to_end_id` of the return itself.

| Instant Payment ID | Required by FedNow? | Required by RTP? |
| :----------------- | :------------------ | :--------------- |
| `end_to_end_id`    | Unsupported         | Yes              |
| `transaction_id`   | Unsupported         | Yes              |
| `uetr`             | Unsupported         | No               |