Tuna mayo is the baseline of the whole tuna family, and the variable it is built on is the bind. Tinned tuna is the constant; what makes this the reference version is that nothing else is added, so the entire sandwich rests on one ratio: how much mayonnaise goes into the drained fish. Get it right and the filling is moist, cohesive, and spreadable, holding together in a clean layer. Get it wrong in either direction and the sandwich fails in a way no bread can rescue, because the bread here is a passive carrier and everything that decides quality has already happened in the bowl before a slice is touched.
The craft is that ratio and almost nothing else. The tuna must be drained properly first, because the water or oil left in the tin is the hidden moisture that later soaks the bread from the inside no matter how the loaf is buttered. Then the mayonnaise is added to the point where the fish just binds and no further: too little and the filling is dry, dull, and crumbles out the open side; too much and it turns to a slick that slides off the bread and greases the crumb. Salt and pepper are worked through, the filling is spread evenly so each bite is the same rather than wet at one end and bare at the other, and the bread is soft and plain because it is doing no work but holding the mix. This is the same lesson the egg sandwich teaches, unforgiving because there is nothing else in the build to distract from a bad bind.
The variations are the rest of the family, and every one of them is this baseline with a single deliberate addition for contrast. Cucumber adds cool water-crisp, onion adds raw bite, sweetcorn adds a sweet pop, celery and pepper add the branded crunch. Each of those changes one variable against this fixed bind and earns its own name by doing so. Each deserves its own article rather than being crowded in here.