A background process doesn't need to interact with UI elements on the screen but rather relies on background processes to pass along information. UIAutomation activities should not be used in these project types. Multiple such processes can simultaneously run on a machine, even if a foreground process is already running.
The type of process is generally dictated by the type of activities it uses (whether or not they interact with UI elements). However, you can specify the type when you create a new process from Studio. By default, all processes are marked as foreground, unless specified otherwise.
UiPath Documentation
Exam Topic: Identify and describe how UI Automation activities and/or Excel activities can be configured to interact with background windows