Explanation
Arma Units are special kind of card you keep Armory Zone (a horizontal zone on the player's right of the Guardian Circle) and once the conditions are met, they can attach themselves to units of the field the give them power, shield, effects, etc. At the end of the specified Durability, those cards are placed face up in the RFG pile and thetefore, cannot be used again.
Once a unit is Armed with an Arma unit, the are placed under the chosen unit so only the bottom of the border is seen. They are not become soul.
You are allowed a maximum of 15 in your Armory Zone. They cannot be placed in the Main Deck or G Zone. Your unit cannot be Armed with more than one Arma Unit at a time, unless through cause and effect, they are allowed to.

Example

Elemental Sword
Clan: Cray Elemental
Material: Element Stone
[Release Restraint] When a unit on your field attacks, arm it with this card]
[Durability] At the end of the battle this unit armed a unit
If this unit is transed , that unit gets [Power]+2000.

Any questions? Ask away.