Most characters are allowed to join certain types of unit over the course of the battle. A character is normally permitted to join units of infantry, beasts, cavalry, monstrous infantry, monstrous beasts or monstrous cavalry. Characters can also join other characters belonging to one the troop types listed above, thus forming an impromptu unit entirely made of characters.
Unless otherwise stated, a character cannot join a unit of monsters (too much danger of being stood on), a unit of flyers (too many ill-disciplined wings buffeting the sky), a unit of chariots (too much danger of being run over), a unit of swarms (too much chance of being eaten) or a war machine (too much danger of being obliterated). Similarly, a character that is itself a monster, or riding a monster or a chariot cannot join other units – it's just too dangerous for the members of the unit in question. A character cannot join a unit that is already engaged in close combat or is fleeing.