When you go to the Processes tab in Task Manager, you see the processes grouped into three categories: App, Background Process, and Windows Process. How does it decide which process goes into which category?
These are terms that Task Manager simply made up. The system itself doesn't really care what kind of processes they are.
If the process has a visible window, then Task Manager calls it an "App".
If the process is marked as critical, then Task Manager calls it a "Windows Process".
Otherwise, Task Manager calls it a "Background Process".
That's all. Nothing fancy. And completely arbitrary.