Salmon paste is the fish member of the store-cupboard paste shelf seen from the jar rather than the bread, and it runs on the same inverted ratio as the rest of that row: the filling is the thing you use least of. Salmon is cooked, reduced, and pounded with butter or oil and seasoning into a smooth, pink, salty spread that keeps sealed at the back of a cupboard, and the whole design follows from that keeping. A paste built to survive months unrefrigerated is concentrated to the point where a thin scrape across buttered bread is already a full mouthful. Spread it like a filling and it is too salty and too strong to eat. Spread it like a stain and it is a sandwich. It is the shelf-stable way to have salmon in the house when fresh fish is neither cheap nor to hand.
The craft, even before any bread is involved, is in what the jar is and how concentrated it has to be. Salmon paste keeps the pink, faintly oily register of the fish but loses every flake in the pounding, so it carries flavour and no texture of its own, and it is salted hard enough to act as its own preservative, which is why the eventual scrape is thin by necessity rather than by thrift. There is no heat in the jar, no second component, no acid; it is a single concentrated thing engineered for shelf life first and the sandwich second. The trade for that keeping is intensity: the same property that lets it sit unopened for months is the property that makes it inedible by the spoonful and exactly right by the smear, which is the founding logic of the whole paste tradition.
The variations are a row of jars rather than a row of recipes. Crab paste carries the same potted idea into shellfish; bloater paste pushes it to the strong, smoked herring end; beef, ham, and chicken paste swap the fish for cured meat on identical engineering. The made salmon paste sandwich, this product read from the bread rather than the jar, is its own consideration. Each deserves its own article rather than being crowded in here.