# Deprecated unique identifier field

The `unique_identifier` parameter to create Transfers was renamed to `Idempotency-Key` in January 2024. The functionality of `unique_identifier` is now available across all Increase API objects.

Learn more about [Increase idempotency](/documentation/idempotency-keys).

We'll still respect the value of `unique_identifier` if your implementation sends that data on POST requests for:

- Account Transfers
- ACH Transfers
- Check Transfers
- Real-Time Payments Transfers
- Wire Transfers

We recommend using the HTTP header based implementation and using Idempotency across all requests you send to Increase.