If you eat two pancakes with butter between like a sandwich, it is a variant of butter sandwich. If you cut up your PB&J such that you're only eating pieces of bread with some spread on them, that is not a sandwich. The bread must act as a container for the contents during consumption, regardless of ingredients.
More generally, the key delimiter for sandwiches is the aspect ratio of the individual items contained by the bread. For something to be a sandwich, all of the major items inside must have one dimension which is substantially smaller than the other two. Sliced meats, bacon strips, cheese slices, sliced vegetables, lettuce leafs, spreads, patties, etc are all valid. Thinly spread peanut butter and jelly between bread is a sandwich, but a bread bowl with a pool of jelly and peanut butter where the depth is comparable to the radius would not be a PB&J sandwich. Flat cheese slice between bread is a sandwich, mozzarella sticks are not sandwiches. Hot dogs are by default not a sandwich, but if you cut the hot dog into thin strips it becomes a sandwich. An oreo is a sandwich but a quadruple stuffed oreo is not. The only commonly accepted sandwich which this definition claims to be a pseudosandwich is the meatball sub.