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 made up of infantry, 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. However, a character is not allowed to join a unit made up of a Troop Type with a higher Unit Strength than his own Troop Type, unless specified.
Unless otherwise stated, a character cannot join a unit of monsters, a unit of flyers (unless they are also a flyer of the same Troop Type), a unit of chariots (unless they are also mounted on a chariot), a unit of swarms or a war machine. Similarly, a character that is itself a war machine, monster, or riding a monster or a chariot cannot join other units (with the exceptions mentioned above). A character cannot join a unit that is already engaged in close combat or is fleeing.