A.R.M. (Automated Reserve Mech) is a kind of unit. In Zero Online, you can have two ARMs with you at most. Then you will have 3 units at the same time. You can set anyone of them as the main unit and the other two will be your ARMs. The ARMs will automatically help the main unit to attack any surrounding enemies. These 3 units can shift each other, but it will take 20 seconds to take another change.
As a unit, the ARM has its own inner gears and outer gears. What's more, it needs to be upgraded, too.

How to Get
When your unit reaches level 100 and your pilot rank reaches Major, you can get your first unit CD from ARMs Dealer (Steel Warship 345,788). And when your unit reaches level 145 and pilot rank reaches
Major General, you can get the second one from him. After that, put the CD in the Hangar, you'll get your ARM.

Operation:
In normal status, your ARMs will attack the surrounding enemies automatically. It is up to you, though. If you click the ARM's level button beside the head portrait, you'll find there are several buttons. You can decide whether this ARM will automatically attack the enemies or not. Only when the unit is set as the main unit can you upgrade it and change its weapons and gears.

More Tips
1. When the main unit is destroyed, one of the two ARMs will take its position automatically. The destroyed unit will be the left ARM or the right ARM. It will recover in 15 minutes. During this period, it can't attack.
2. The ARM has its own potency. The potency will be directly added to the main unit. Your overall potency is composed of the main unit's and the two ARMs', which will be displayed in the interface.
3. If any of your ARMs is destroyed, your overall potency will by weakened by 3.