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What is the cheese, lettuce, tomatoes, etc. in a sandwich called collectively? For example, I know that mushrooms, cheese, peperoni on a pizza are called toppings.
They would be toppings in AmE.
In BrE, a topping would not be covered by a second slice of bread- a filling would.
"Fillings" might be used with something totally enclosed, like a calzone. Even then, it wouldn't be weird to call them "toppings," since they are essentially the same choices as the toppings for a pizza. And you would normally buy a calzone from a pizza place.
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It would appear to be another of those differences between the variants.
IMO, we do not say "filling" for sandwiches because "filling" is what goes in pies. And by pies, I mean fruit pies, not some weird meat-filled thing.
There's also condiments, but those are typically just sauces, like ketchup, mustard, or mayo.