When VTAM® invokes the USERVAR exit because a USERVAR is being updated, the exit overlays the value stored in the UVCB for that USERVAR with the new value. If there is no UVCB for the USERVAR, the exit attempts to create one. If the exit does not allocate the UVCB successfully, it sets the return code to 0 and returns control. Likewise, if any of the processing required to update the USERVAR fails, the exit sets the return code to 0 and the exit returns control.