The Portion Model field (visible when Order: Subscription > Meal Plan: Portion Size per Category is Yes) has two values:

• Preset — one portion size for the whole plan. You pick the plan's Portion Size, and customers choose their portion at checkout from your active portion sizes.
• Customize (a Custom plan) — you define, on each price row, how many meals of each portion size every category includes (e.g. Breakfast: 5 × Small; Entrees: 5 × Large, 5 × Regular). Customers don't choose a portion — the mix is baked into the bundle.

The "Add new" pop-up on the plans list explains both choices, and the plan form's help text reads "Custom plans set portions per category on each price; the customer isn't asked to pick a portion."