Creates or updates custom field values for many subscribers in one request. Each entry in custom_field_values pairs a subscriber_id with a subscriber_custom_field_id and the value to set. Requires OAuth authentication.

Requests with 100 or fewer values are processed synchronously (200); larger requests are processed asynchronously (202) — provide a callback_url to be notified on completion, and see Bulk & async processing for how async requests behave. Returns 413 when the request would exceed the account's data limit for enqueued bulk requests.

The custom fields must already exist — entries referencing an unknown subscriber_custom_field_id are rejected individually with a Custom field does not exist error in the response's failures list, while the rest of the batch is still processed. See List custom fields and Bulk create custom fields.