The region size is the amount of storage in the user region available to the job, started task, or TSO/E user. The system uses region size to determine the amount of storage that can be allocated to a job or step when a request is made using the STORAGE or GETMAIN macros and a variable length is requested. The region size minus the amount of storage currently allocated, determines the maximum amount of storage that can be allocated to a job by any single variable-length GETMAIN request.