You can customize your formations during play and make different kind of formations for different tasks. Each formation can consistent of up to 4 types of unit types.
Due to this system, it is not possible to put light infantry and heavy infantry in the same formation. It is possible to put light infantry, humvees, mortars and light tanks in the same formation. The best mix of unit types will depend on the scenario and your tactics. The reason to mix unit types is so that they can work together. Mainly:
Land Mobile
Mobile and armour types can tow or carry infantry and artillery types. Thus increasing the number of movement points of the formation.
Land Support
Artillery types with range=1 participate in a supporting role in land battles. They add to the firepower of the unit, and can only be destroyed if the unit is forced to retreat. They are overrun on the moment they are left without infantry or armour accompaniment. The same goes for mobile unit types, which are purely supportive in the movement and recon area, but bring no combat value whatsoever.
Armour Cover
Infantry will take cover inside armour during battle if possible. Infantry inside or behind vehicles still participate in combat, but they can only be destroyed after the armour they are covered by is destroyed.
Long Distance Artillery
A land formation which has long distance artillery (distance>1) is considered an artillery formation. Infantry and Armour added to the unit can only be used defensively. All attacks the formation participates in will be through indirect fire.
Escort Role
If air units are interdicting, transferring, airlifting of bombing the fighters in the same formation will always escort the other aircraft (transporter or bomber) and engage enemy intercepting fighters. Only after all escorts are destroyed will the transporters and bombers have to fight with the enemy escorts.