Body is an ARRAY of group updates identified by group_key. translations is required on every item (all active languages of the site). Validation is all-or-nothing (invalid items only are returned, nothing is updated; an unknown group_key stops validation with CD0054). When customs_data is sent, associations are synchronized in a transaction (adds, removes, reorders); removing a custom data holding product values fails with CD0057. Business errors are returned with HTTP 200 and success: false; each message is prefixed with its code (CDxxxx - ...).