POS Categories (labeled "Primary Categories" in the left menu) are the tabs a cashier taps across the top of the POS item grid to filter what's shown (e.g. Drinks, Sandwiches, Sides) — an alternative grouping to your regular Meal Categories. They're managed at Admin > POS > Categories (/admin/pos/categories), a simple list with just a Name and an Is Active toggle per category. The same table also feeds the Grab & Go category filter, which is why the left-menu link only shows up when both the Retail and Grab & Go modules are active — note, though, that actually opening the page also requires the POS module itself (and the "List POS Devices" permission) to be enabled, so it's possible in principle to see the menu link without being able to load the page if only Retail + Grab & Go (not POS) are active.