IBM SC33-1683-02 - Manual (Page 314)

Tracing a task-related user exit program
CICS outputs a trace entry just before control is passed to the task-related user exit
and just after returning from the exit. You can control these trace entries using the
RI option of the CETR trace control transaction or the EXEC CICS SET
TRACETYPE command.
adapter administration
282 CICS TS for OS/390: CICS Customization Guide