Convention for making POST-style creation idempotent. The sender generates (or retrieves) a unique token representing the intent of the operation and attaches it to the request (as a field or header). The receiver indexes stored resources by this key; on a duplicate request with the same key, it returns the existing resource instead of creating a new one. Same key = same intent, so retries never create duplicates. Response conventions vary (success, 409 Conflict, 422 Unprocessable) and consistency within a team matters more than the specific choice.