For the BEFORE COMMAND, AFTER COMMAND, AFTER THREAD INITIALIZATION and BEFORE THREAD TERMINATION exits (XLBCEUSR, XLACEUSR, XLATIUSR, XLBTTUSR), this is the address of a private data area given to each of these exits on a per thread basis. This area is not shared with any other exit or any other thread. It is defined in the PRVTWORK parameter of the DK1EXDFN TYPE=ENTRY macro.
AFTER REGION INITIALIZATION and BEFORE REGION TERMINATION exits (XLARIUSR, XLBRTUSR) do not use parameter 3.