The Xnet Numeric Codes are documented in the 'CA Database Management Solutions for DB2 for z/OS - 19.0' manual in the following link:
This message is logged by Xnet modules when the service that was requested could not be successfully performed. In such a case, the Xnet modules will return a non-zero return code and an explanatory reason code.
Specifies the Xnet return code value (decimal).
Specifies the Xnet reason code value (decimal).
All occurrences of non-zero return codes from Xnet modules are logged by this message. However, there are cases where the non-zero return code is an expected or acceptable condition. For example, during the agent connection process, Xnet tries to “locate” the control structure for that agent and Xnet expects this to be “not found”. If it was found, that would mean that an agent with that name is already connected to Xnet and the agent that is trying to connect must be rejected.
For the above sample XNTTASK xntXNT101E Xnet IXNTALPD returned error rc 204 rs 285 it indicates:
rtncode 204: Xnet_RC_Xnet_request_failed
rsncode 285: Xnet_RS_Definition_not_found