Defenseless on their own, Armored Personnel Carriers (APCs) instead support an army by transporting Foot Soldiers and refilling Fuel and Ammo stocks of adjacent units.
The most notable function of the APC is the loading and transporting of one Infantry or Mech unit at a time, offering them more mobility in open terrain. While loaded, the APC will also protect the unit it is carrying from attack, with the APC taking full damage instead. If the APC is destroyed while a unit is currently loaded in it, the loaded unit will be instantly destroyed along with the APC. Moving the APC will not actually end its turn, and it can be loaded or unloaded as many times at any point during your turn as long as there are adjacent spaces available for foot soldiers to be dropped. APCs can still only make one move action per turn, however. When being unloaded, a unit will not be able to take any more actions for the turn, so it is not advisable to drop units into vulnerable positions.
The secondary function of the APC is to resupply any adjacent units of the same army. Doing so will completely refill the Fuel and Ammo stocks of any adjacent units. APCs will automatically resupply adjacent units at the start of your turn, and can also be manually told to resupply in the action menu. Resupplying will end the APCs turn as if it were being told to wait.
APCs are also decent blocking units as they do not lose effectiveness as they lose HP, and are difficult for Infantry to destroy. They are much too expensive to be used solely for this role, but can be useful if the APC has fulfilled its primary role already and can block enemy Infantry from capturing a property.
Enemy APCs can prove to be a nuisance if they are blocking properties from being captured, but are otherwise harmless. Unless it is absolutely necessary to gain control of a space that an APC is blocking, APCs can generally be ignored in favor of more dangerous enemies. If an opponent is fielding Stealths alongside APCs, it may prove worthwhile to destroy any APCs to limit the Stealth's mobility before it runs out of fuel.