The Pan Bagnat Traditionnel is the version defined by its rules rather than by any single ingredient. It follows the strict Niçois specification: a round country loaf, olive oil and red-wine vinegar, tinned tuna and salt-cured anchovy, hard-boiled egg, tomato, raw onion, small black olives, sometimes a leaf of basil, weighted and left to rest. What separates it from a looser pan bagnat is what the strict version refuses. No lettuce. No cooked vegetables, which means no boiled potato and no blanched green bean however much they belong on a plated salade niçoise. No mayonnaise-bound tuna. The local olive oil is not a garnish but a named requirement. This is the reading where the exclusions carry as much weight as the inclusions.
The orthodoxy is coherent rather than merely proud, and the soak is why. Every excluded ingredient is excluded because it would not survive an hour under weight: lettuce wilts, boiled potato turns the crumb to paste, mayonnaise breaks. What remains is exactly the set of components that improves while it sits, the raw vegetables giving up some water, the oil and vinegar working down into the bread, the tuna staying in flakes because it went in plain. The bread has to be the sturdy, faintly tart Niçois round; a baguette is the standard outside mistake and produces a sandwich that wets through within minutes. Cut the strict version open after its rest and the crumb is saturated and unified, never crisp, which is the intended state and not a fault.
Because the traditional reading is the constraint itself, its variations are small and stay inside the spec: the proportion of tuna to anchovy, more tomato when summer fruit is loud, a leaf of basil present or absent, the exact olive used. None of these breach the rules, which is the point of calling the version traditional in the first place. The other readings that loosen the spec, leaning tuna-forward, anchovy-forward, or dropping the fish entirely, are the deviations this one is measured against. For the full account of the soak, the bread, and where these Niçois rules come from, see Pan Bagnat.