Selects the representation of custom_fields on items, for both the request body and the response.

• default — object keyed by custom field definition id (UUID).
• legacy — the {field_name, field_value} array tied to custom fields collections.

Most accounts do not need this header: an account that has not moved to item custom fields always gets legacy, and an account that has moved and no longer uses collections always gets the object representation. The header matters only while both are available for the account, where legacy is the default and default opts into the object representation.

Requesting a representation the account does not have returns 422, as does sending a custom_fields body whose shape does not match the representation in force, mixing both shapes in one request, or combining custom_field_collection_id with the object representation — that field belongs to the legacy representation and is left out of responses entirely under the object one. Responses vary on this header.