The DFASY, RESP, and SCIP EXLST exit routines can be in an exit list that is associated either with an ACB (identified by the EXLST operand of an ACB) or with a NIB (identified by the EXLST operand of an NIB). VTAM® uses ACB-specified exit routines for all sessions with the program represented by the ACB. VTAM uses NIB-specified exit routines only for each session whose NIB specifies the exit routine when the session is established. With one exception, the DFASY, RESP, and SCIP exit routines identified in an NIB exit list are scheduled instead of corresponding exit routines identified in an ACB exit list. This exception is in the processing of a BIND request; the ACB-specified SCIP exit routine is always scheduled in this case. For details on how VTAM classifies DFASY and RESP input, and decides to schedule a RESP or DFASY (NIB or ACB) exit routine instead of completing RECEIVE, see Figure 1 through Figure 4. Several sessions can share the same list of DFASY, SCIP, and RESP exit routines, or the list can be unique for each session.

Figure 1 shows two sets of NIB-specified exit routine addresses. When input from the session associated with NIB1 arrives, the appropriate EXLST1 exit routine is scheduled. When input from the session associated with NIB2 arrives, VTAM checks EXLST2 for the appropriate exit routine. If no exit routine is specified (which in this example would be true if the input is a response, because EXLST2 has no RESP entry), VTAM satisfies any pending RECEIVE macroinstructions or checks for an ACB-specified exit routine address in EXLSTA. When input from any other session arrives, VTAM uses EXLSTA.