IBM SC33-1683-02 - Manual (Page 114)

Example program
CICS supplies, in CICSTS13.CICS.SDFHSAMP, an example program,
DFH$XTSE, that shows how to modify fields in the command-level parameter
structure passed to EXEC interface exits. DFH$XTSE is listed on page 807.
file control EXEC interface API exits
82 CICS TS for OS/390: CICS Customization Guide