The priority rule in foil and sabre: on a simultaneous hit only the fencer who started the action scores. Épée has no such rule.
This is the one concept that loses almost every spectator: both lights come on, and still only one fencer scores. In foil and sabre, right of way applies — the hit is awarded to whoever started the action, and simply thrusting back at the same time is not enough. Épée has no such rule: there it comes down purely to whose point lands first, regardless of who began. That single difference is why the three weapons play so differently — épée rewards patience and low-risk fencing, while foil and sabre run on initiative. In those two weapons the parry is therefore not a flourish but the way you take the initiative back. And here is what tends to get missed: right of way is a referee's decision, not something the equipment shows. The box only reports that a hit landed — never who is awarded the point for it.