My too-many years laboring in restaurant kitchens might come in handy here:
Topping is any sort of food that coats, sits on top of or TOPS another food...
Meat might be topped with a sauce, a desert might be topped with melted butter and sugar, enchiladas are topped with cheese.
Heated self deck warms two 35lb. boxes of topping for immediate switch over from first bad to second.
The self-heated deck warms two 35 lb. boxes of topping for immediate switching over from the first batch to the second batch.
(A Deck is a type of food-warmer, self-heating means it comes with its own heating, you don't have to put it on a stove, and Topping here refers, I believe, to food topping to coat food... melted cheese, perhaps, sauces, desert toppings, etc. They are bragging of their ability to have two batches, the current and the backup, heating at the same time... rather than have one batch hot and the other one stone-cold until you the first one runs out... hence you ability to ditch the empty, have the second already heated, with no interruption or wait time for the second to heat.)