Source: Warhammer Fantasy: 7th Edition

Weapons and Units
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It is usual for all the models in a unit to carry the same weapons. So, a unit will be a unit of Spearmen, a unit of Halberdiers, a unit of Crossbowmen and so on. It is acceptable for a unit to include a minority of models that are differently armed for the sake of a varied and interesting appearance, but the whole unit still counts as being armed as the majority. Where models are varied in this way, it is important that the overall appearance
of the unit is not misleading.

All troops and characters are assumed to carry hand weapons, such as swords, axes, clubs, maces and comparable weaponry. In addition, some troops carry another weapon such as a spear, lance or halberd. At the start of the first turn of a combat, each such unit can choose which of their weapons to use (the entire unit must use the same weapon, but characters can always choose separately). Whichever weapon they use must then be used for the entire combat. For example, troops armed with spears and shields may opt to fight with their swords if they want to benefit from the defensive bonus conferred by fighting with hand weapon and shield, but must then fight with swords for the duration of the combat, even if it lasts many turns, and cannot switch to their spears.

Next - Special Weapon Rules