It chooses which category list the Grab And Go screen of the admin checkout, when running in POS mode, uses for its side menu and filtering: Meal Categories (default) uses your regular meal categories, while Primary Categories switches to the separate POS Categories you set up for point-of-sale screens, and meals are filtered by their assigned POS category instead. Pick Primary Categories when your till staff work with a simplified POS category set. The POS Menu module has the same choice, set separately.