Job queues are work entry points for batch jobs to enter the system. They can be thought of as "waiting rooms" for a subsystem.
A number of factors affect when the job is pulled off the job queue into the subsystem, such as the job priority on the job queue, the sequence number of the job queue, and the maximum active jobs. When all of these factors work together, the job will be pulled off the job queue to start running in the subsystem.
When the job enters the job queue, it is available to a subsystem that has the job queue allocated to it. Because subsystems can have more than one job queue feeding into them (however, job queues cannot feed into more than one subsystem), a sequence number in the subsystem determines when the subsystem processes a job queue. The subsystem looks at the sequence number of the job queue before the job priority of the jobs in the job queue. The subsystem uses the priority on the job queue to determine when a job can enter relative to other jobs on the job queue. The job priority and the maximum active jobs determine when a job enters the subsystem.