Plan Categories (modal "Filter meals for this subscription") is a filter: tick the meal categories the customer is allowed to choose from; leave all unticked for no filter. It narrows the menu shown in the meal picker and in auto-selection. Categories per day is a quota: it says how many meals of each category must be picked each day (for example 1 Breakfast + 2 Lunch). Both exist at subscription level (Renewal Settings) and per delivery date (Subscription Dates panel: Plan Categories and Categories/day), so a single day can differ from the plan default.