This is a short quote from the MSDN
The *_PRIORITY_CLASS values affect the CPU scheduling priority of the process. For processes that perform background work such as file I/O, network I/O, or data processing, it is not sufficient to adjust the CPU scheduling priority; even an idle CPU priority process can easily interfere with system responsiveness when it uses the disk and memory. Processes that perform background work should use the PROCESS_MODE_BACKGROUND_BEGIN and PROCESS_MODE_BACKGROUND_END values to adjust their resource scheduling priorities; processes that interact with the user should not use PROCESS_MODE_BACKGROUND_BEGIN.
If a process is in background processing mode, the new threads it creates will also be in background processing mode. When a thread is in background processing mode, it should minimize sharing resources such as critical sections, heaps, and handles with other threads in the process, otherwise priority inversions can occur. If there are threads executing at high priority, a thread in background processing mode may not be scheduled promptly, but it will never be starved.