Open the meal in Admin > Meals, edit it, and (when the POS module is on) use the Primary Category dropdown to pick one of your POS Categories — this is a separate field from the meal's regular Categories multi-select. A meal can only belong to one Primary Category at a time. Picking "--" leaves it uncategorized: on the POS Terminal it will still show up under the catch-all All Meals tab (which lists every meal regardless of category), but it won't appear under any of the named POS Category tabs.