For the BEFORE COMMAND, AFTER COMMAND, AFTER THREAD INITIALIZATION and BEFORE THREAD TERMINATION exits (XLBCEUSR, XLACEUSR, XLATIUSR, XLBTTUSR), this is the address of the data area shared by each of these exits when they are part of the same task (thread). This work area may be used for passing information between these exits. It is defined in the SHRDWORK parameter of the DK1EXDFN TYPE=INITIAL macro.
AFTER REGION INITIALIZATION and BEFORE REGION TERMINATION exits (XLARIUSR, XLBRTUSR) do not use parameter 3.