The Available Portions dialog lists the meal's default portion plus every portion size enabled on that meal, grouped by portion type. With the Catering module you get the catering portion sizes (trays, per-person catering portions) and, if a Portion Size Default Catering is configured, that catering portion with its catering price; with the Meals-by-unit module you get unit portions; with both you see all of them. Each portion becomes its own line with its own quantity and price. Portion sizes are created under the meals/portion-size settings.