MPLAB® Harmony system services are responsible for managing shared resources so that other modules, such as drivers, middleware, and applications, do not conflict on shared resources. For example, if the TCP/IP, USB, and Graphics stacks attempted to concurrently use the Timer2 peripheral to perform some periodic task, they would likely interfere with each other.
However, if they used a timer system service, it is the responsibility of the system service to keep the separate requests from interfering with each other. The timer service can be configured as desired for a specific system (for example, you decide to use Timer3 instead of Timer2) isolating the necessary changes to the configuration of a single module and preventing potential conflicts.
The use of a system service is very similar to the use of a device driver, except that a driver normally requires the caller to "open" it to create a unique client-to-driver association. A system service does not normally require the caller to open the service before using it because system services are frequently shared by many clients within the system.