The control program creates one task in the address space as a result of initiating execution of the job step (the job step task). You can create additional tasks in your program. However, if you do not, the job step task is the only task in the address space being executed. The benefits of a multiprogramming environment are still available even with only one task in the job step; work is still being performed for other address spaces when your task is waiting for an event, such as an input operation, to occur.
The advantage in creating additional tasks within the job step is that more tasks are competing for control. When a wait condition occurs in one of your tasks, it is not necessarily a task from some other address space that gets control; it may be one of your tasks, a portion of your job.
The general rule is that you should choose parallel execution of a job step (that is, more than one task in an address space) only when a significant amount of overlap between two or more tasks can be achieved. You must take into account the amount of time taken by the control program in establishing and controlling additional tasks, and your increased effort to coordinate the tasks and provide for communications between them.