In normal operation, every background work process processes jobs of every priority.
You can, however, reserve some of the configured background work processes as desired for high priority jobs; that is, class A jobs. The reservation of work processes for class A jobs does not reserve any particular work processes. Rather, it ensures that a particular number of background work processes are kept free for class A jobs whenever possible.
Jobs of classes B and C can only be started if the defined number of work processes remains free for possible class A jobs, as shown in the figure below.

This option allows a choice between two prioritization strategies for your background jobs:
Critical track (class A) and two-level priority (classes B and C): If you have mission-critical jobs whose immediate execution must always be ensured, you can set up class A as your "critical jobs" category. In this case, reserve a work process for class A jobs and assign class A only to these mission-critical jobs. In this scenario, class B and C jobs are very unlikely to block a class A job.
Three-level priority for allocation of free work processes: You can also decide to run with a straight three-level priority scheme for allocating free work processes. In this case, reserve no work processes for class A. If the class A workload is small, or bottlenecks rarely occur in background processing, that is, at least one background work process is usually free, the reservation of a work process for class A jobs probably provides no advantages. In this case, reservation will simply mean that a work process is seldom used.
To set the number of reserved background work processes for class A, you can define an operation mode in transaction RZ04 (CCMS: Edit Operation Modes and Instances) and maintain the work process distribution for this operation mode. When doing so, you have the option of reserving work processes.
Hint
If you want to use the function for reserving background processes for class A jobs, it is usually enough to define one background work process for class A jobs in each system. We recommend that you do not reserve more than one background work process for processing class A jobs for each application server.