Every topping is off, light, regular or extra. A recipe with eight toppings gives you eight portions. Off frees one, extra costs two, and light still costs one — it's a regular portion with less on it. Stay at or under the recipe's count and you pay the recipe price, whatever that is at your store.
Confirmed on a real receipt: going light on a topping does not free up room for anything else. What it does buy you is roughly half the calories, fat and sodium of that topping at no change in what you pay. Useful when you're already balanced and want the pizza to be lighter.
Papa Murphy's publishes numbers per finished pizza, not per topping, so this is an additive model built from portion weights and ingredient composition, calibrated against the published Cheese, Pepperoni, Cowboy and Papa's Favorite slices. Calories, fat, sodium, carbs and protein land within about ten percent. A guide, not a label.
Whether the register nets portions across different toppings is point-of-sale behavior, not published policy. Switch the swap rule to "per item" to see the answer if your store charges for every increase, and check the runner-up before you commit.