During MT06 I’ve been playing crossbowmen on the assumption that their medium armour class applied also to their ranged fire. In other words enemy heavy cavalry or foot (haven’t seen any heavy foot) ignore one sword from crossbow fire.
I can’t find any definite guidance on this although the rule book (p.12), written before any units existed which hit on swords at range, states that armour class only applies in close combats.
as far as I know, Armour difference does not affect ranged fire. This makes Crossbows very deadly
Anecdotally, participants in the 3rd Crusade mention armoured Crusaders with 10 arrows lodged in their armour without any apparent hinderance, where as Crossbow bolts penetrated both riders and their horse.