When you code VSAM user exit routines, you should have available z/OS DFSMS Macro Instructions for Data Sets and z/OS DFSMS Access Method Services Commands and be familiar with their contents.

A user exit that is loaded by VSAM will be invoked in the addressing mode specified when the module was link edited. A user exit that is not loaded by VSAM will receive control in the same addressing mode as the issuer of the VSAM record-management, OPEN, or CLOSE request that causes the exit to be taken. It is the user's responsibility to ensure that the exit is written for the correct addressing mode.

Your exit routine can be loaded within your program or by using JOBLIB or STEPLIB with the DD statement to point to the library location of your exit routine.