Every resource you create takes two fields of your own. KetaPay stores them and never acts on them.

external_reference

Your own id for the object, such as an order or project id. Up to 255 characters, indexed, and filterable on lists.
The escrow created from a session keeps the session’s external_reference, and it appears on every webhook event about it, so you can match events to your records without storing KetaPay’s ids.

metadata

Up to 20 key-value pairs, for anything else you want back later.
Don’t put sensitive data in either field: no card numbers, passwords or government ids.