Crab and lemon is one of the lightly dressed crab sandwiches, and what separates it from its siblings is the acid. Fresh white crab meat is bound in a lemon mayonnaise on brown bread, so the dressing carries a clean, bright citrus note worked all the way through rather than squeezed over the top at the table. The lemon is not a wedge on the side of a mild filling. It is dissolved into the bind so every forkful of crab arrives with the same sharp lift cutting the meat's sweetness. That integration is the whole sandwich. Take the lemon out and you have plain bound white crab, which is gentler and less defined.
The craft is calibrating the acid against the bind and the meat without breaking either. Too much juice and the mayonnaise slackens, loosens, and weeps into the bread; too little and the citrus reads as nothing against the fat and the sweetness. The fix is to lean on zest, which carries the lemon oils and aroma without the water, and to use only enough juice to brighten, so the dressing stays thick enough to hold the crab in a cohesive layer. The crab is white meat for a reason here: it keeps the filling pale, delicate, and sweet, the exact register lemon was chosen to sharpen, and it is folded gently so it holds its flake rather than collapsing to paste. The bread is soft, plain brown, and buttered to the edges so the crumb is sealed against a filling that is, by design, slightly wet, and so nothing argues with a sandwich built on lightness and acid. Cut thin and not overfilled, it stands on the crab being fresh and the lemon being measured.
The variations are the rest of the dressed-crab shelf, each defined by the single note worked against the meat. The plain crab sandwich runs white and brown meat with only butter and a squeeze of lemon; crab and chive swaps the acid for a mild onion herb in the bind; crab paste carries the same shellfish to the concentrated, shelf-stable end of the larder. Each deserves its own article rather than being crowded in here.