Many models carry two or more combat weapons, giving them a choice of which to use upon entering the fray:
If a unit that is engaged in combat is equipped with two or more combat weapons, you must choose which weapon it will fight with when its combat is chosen during Step 1.1 of each and every Choose & Fight Combat sub-phase.
The entire unit, including command, must use the same weapon (with the possible exception of units with the Motley Crew special rule). Characters can always choose separately – they are not obliged to use the same weapon as a unit they have joined.
If a unit champion has the option to be equipped differently to the rest of their unit, they can choose which weapon they will use separately.