The Jambon-Beurre Tradition is defined by its bread, and specifically by a legal definition of bread. The baguette de tradition française is a regulated category: it may contain only wheat flour, water, salt, and leavening, with no additives and no freezing of the dough at any stage. A baguette made to that standard bakes differently from an ordinary one. It develops a darker, more blistered crust, an irregular open crumb with large uneven holes, and a flavor with more depth from the longer fermentation. This sandwich is the jambon-beurre built specifically on that loaf, and the bread is the whole reason it carries a different name.
The filling is the same restrained trio: barely-salted butter spread thick, slices of jambon de Paris cut to order, nothing else. What changes is how that trio reads against a better crust. The tradition baguette shatters at the first bite rather than tearing, and the open crumb gives the butter somewhere to settle instead of compressing into a dense band. The longer-fermented bread carries a faint tang and a nuttiness that the ham and butter no longer have to compensate for. A plain baguette can make a fine sandwich; this one lets the bread be the most interesting thing in it.
The craft consideration is timing, which the tradition loaf makes stricter rather than looser. Because it contains no additives to extend its life, it stales fast, going from perfect to past it within a few hours of leaving the oven. A boulangerie that bakes in batches through the day can offer this sandwich at its best; one that bakes once cannot. The butter has to go on a loaf still close to the oven, and the sandwich has to be eaten soon after, not held in a case until afternoon.
Variations are mostly a matter of which charcuterie or cheese the better bread is asked to carry: a dry-cured ham, a slice of young Comté, a country pâté, each benefiting from a crust with more character underneath it. All of them are the same move, the Jambon-Beurre built on a loaf held to the artisanal standard, and the bread doing more of the work than usual.