When an application program issues a VTAM® macroinstruction or when an application program exit routine is invoked by VTAM, that particular part of the program is explicitly or implicitly associated with an operating system task. A session can also be associated with a task; the task associated with a session is either the task associated with the OPNDST or OPNSEC macroinstruction used to establish the session or, for LU 6.2 applications, the task that opened the ACB. Task association is used to isolate errors to a particular task, as described in Isolation of errors. It is also used to determine which user exit queue is used to dispatch certain asynchronous exit routines; user exit queues are discussed in Serialization of execution.