For the communication between selfcheck kiosks and Alma, the SIP2 protocol is needed.
The full protocol can be found here: https://developers.exlibrisgroup.com/wp-content/uploads/2020/01/3M-Standard-Interchange-Protocol-Version-2.00.pdf

Unfortunately, Ex Libris does not support all fields that would be available in the protocol (marked as 'not supported' in this document): https://developers.exlibrisgroup.com/alma/integrations/selfcheck/sip2/

Please make the whole SIP2 protocol available for Alma.