When something is filled, it's crowded, packed, or stuffed. A filled donut is crammed with either jelly or delicious custard.
A filled concert venue is at capacity, with every single ticket sold and just about every seat taken. And a filled calendar doesn't have any time for more appointment slots. This adjective is sometimes used interchangeably with "full," but it's more likely to describe a food item that's stuffed full of some kind of filling, from filled pasta shells to filled pastries, or to substitute for "fulfilled," like your filled online order for cat food.