On a Preset plan the Portion Size dropdown lists your active meal-prep portion sizes plus a blank default option (labeled "Normal" out of the box — this default label is an account-level setting Sprwt manages for you, it is not something you can change yourself from your Client Config screens). It sets the portion the plan is built around; on the website, customers see a portion selector and the plan's description for that portion. If the portion size you chose is later deactivated or deleted, the plan stops appearing on the website.

Add/Edit Portion Description opens a pop-up whose text ("This description will show on the Subscription creation page") is displayed under the portion buttons on the customer page.

On a Custom plan the Portion Size field is hidden, because portions are defined per category on each price row instead.