When VTAM® establishes sessions between application programs and LUs, VTAM keeps track of the LUs that are currently in session with a generic resource application. VTAM can distinguish which applications are acting as generic resources, and is aware of the affinity that is created between an application and any LU that has established a session with it. Each time the LU initiates a session using the generic resource name, VTAM establishes the session with the same generic resource application until the affinity is terminated.

The affinity between an LU and an application program is controlled by either VTAM or the application program; the controlling party owns the affinity. LU 6.2 application programs control this affinity if the session uses sync point services, limited resource support, or if the application specifies OPTCD=GNAMEADD on the SETLOGON macroinstruction with AFFIN=APPL in the NIB. An LU 6.2 application program can also override the ownership on a specific LU basis using an optional APPCCMD macroinstruction keyword, LUAFFIN=APPL|NOTAPPL. This optional keyword applies to the ALLOC, PREALLOC, and OPRCNTL CNOS APPCCMD macroinstructions. (Refer to those macroinstructions for additional details.) When the affinity is owned by the application program, the CHANGE macroinstruction must be used to cause VTAM to terminate the affinity. The LU 6.2 application can terminate this affinity between the LU and an application that is a member of a generic resource. When VTAM owns the affinity, it will be terminated when the session is ended, in most cases.

For more information about the CHANGE macroinstruction, refer to z/OS Communications Server: SNA Programming.