These messages show the return information from the z/OS® UNIX System Services used to implement the REXX sockets API when SOCKDEBUG is specified in the TCPIP DATA data set. The BPXxxxx service call is service and the name of that service is socket_call.

For example: EZY1421I BPx1SOC SERVICE CALL Socket call: socket

These services, and the associated return information, are documented in z/OS UNIX System Services Programming: Assembler Callable Services Reference.