IBM SC33-1683-02 - Manual (Page 640)

A general note about user-written programs
On return from any user-written program, CICS must always receive control in
primary-space translation mode, with the original contents of all access
registers restored, and with all general purpose registers restored (except for
those which provide return codes or linkage information).
For information about translation modes, refer to the IBM ESA/370 Principles
of Operation manual.
608 CICS TS for OS/390: CICS Customization Guide