Ammo and fuel are very important supplies for any type of units. If an unit has already run out of its ammo, it is incapable of continue the fights. Running out of fuel will incapacitate your vehicles. Remember that some of the vehicles use several types of ammo. For example, tanks use AT ammo, fragmentation ammo and machinegun rounds. Additionally, the more powerful projectiles, e.g. Katyusha rockets, or heavy artillery ammo use up more supply points in delivery vehicles. Keep that in mind. Possibly, instead of supplying artillery with ammo, which uses up its supplies in the blink of an eye, it is better to replenish ammo for several tanks, or an infantry unit? Small icons of rounds and gas can inform you of such insufficiencies in supplies.
For the majority of missions, you will be fully stocked with ammo and fuel for your units, as well as with a delivery truck. Such a truck will be satisfying demands of units you choose. All you have to do is select the vehicle and press the right mouse button on the unit that needs restocking. The delivery truck will then replenish supplies of the unit automatically, if there is an unit around that is in need of that. Most of the times, it is artillery that will need resupplying All you have to do is park the truck near them and they will be automatically restocked. In nearly every mission, there are locations where you can seize, or recapture enemy supply units. Sometimes, you will find allied, or enemy restocking points. You recognize them by the icon of a stack of crates and barrels, and a flag specifying their allegiance. All you have to do is command your soldiers to get there, in order to seize the storage. Such storages can either load their supplies onto trucks, or stock nearby units. As of now, there is no option to carry supplies from one tank to another, or to return supplies to the truck. Keep that in mind when you are short on fuel, or your tanks will stop and you will be unable to use them in the main assault.
Vehicles and tanks cannot replenish their resistance points to the full amount. The chance that somebody is carrying metal plates to repair tanks in the battlefield is rather thin. The best you can do is restore 50% of their original resistance, if only you have a special skill of support doctrine. Your repair trucks can repair critical damage to your vehicles, e.g. damaged engine or thread. When a vehicle needs repairing, exclamation mark is displayed next to it, which is when you know to send your truck to such a unit. Just remember, trucks are vulnerable to enemy fire and they often become useless after a single shot.
Seizing vehicles is easy, if they are abandoned. Any soldier can capture a vehicle, but enemies, if they are nearby, will immediately start firing at you. This is why you should check the area beforehand and capture the vehicle. After that, restock it. Delivery trucks can deliver ammo for all factions, which is why you do not need to worry that a specific type of ammo will not work with captive vehicles. Most of the times you will be finding damaged and abandoned vehicles, but sometimes you manage to kill the crew of the enemy vehicle. Sometimes, there is no point in capturing such a vehicle, especially when its resistance is low. Your infantry will be more effective out in the field, rather than inside a heavily damaged tank.
The easiest ones to capture are enemy cannons. All you have to do is attack them with riflemen. Leave mortars, rocket launchers, etc. behind (if you want to, you can take vehicles fitted with machineguns along, but they have to flank the enemy). Cannon shave front armor plate, so shots fired will hit the cannon itself, instead of the crew. Perform a flanking attack, or forward a part of the unit, as a decoy. Approach the enemy as quickly as possible and attack the crew. This way, you will seize an undamaged cannon that can be immediately deployed against the enemy. If you uise tanks or artillery fire instead, you stand a higher chance of damaging the cannon itself. Apart from that, AT shots are ineffective against infantry, so they will sustain smaller losses.