A global user exit point (sometimes referred to simply as a "global user exit") is a place in a CICS® module or domain 1 at which CICS can transfer control to a program that you have written (a global user exit program), and at which CICS can resume control when your exit program has finished its work. You do not have to use any of the global user exits, but you can use them to extend and customize the function of your CICS system according to your own requirements. For a complete list of the global user exit points, see Table 2.
Each global user exit point has a unique identifier, and is located at a point in the module or domain at which it could be useful to do some extra processing. For example, at exit point XSTOUT in the statistics domain, an exit program can be given control before each statistics record is written to the SMF data set, and can access the relevant statistics record. You might want to use an exit program at this exit point to examine the statistics record and suppress the writing of unwanted records.
Global user exit support is provided automatically by CICS. However, there are several conventions that govern how you write your exit program, which are described in Writing global user exit programs. Also in that section is a list of the standard parameters that the calling modules and domains pass to an exit program, and some information about returning values to the caller.
Because global user exit programs work as if they were part of the CICS module or domain, there are limits on the use you can make of CICS services. Most global user exit programs cannot use EXEC CICS commands. By contrast, most global user exit programs can invoke some CICS services using the exit programming interface (XPI). For more information, see Using CICS services.