Pavise

A pavise is a large shield which each Crossbowman can prop up in front of him.
To represent this, Crossbowmen equipped with pavises have an additional +2 Armour save versus normal and magical missile attacks.
They do not get this bonus in close combat.
So a Crossbowman in heavy armour with a pavise gets an Armour save of 3+ versus missile attacks, but only an Armour save of 5+ in close combat.