Official Mechanic:Drop Zone

"Drop Zone" (Japanese: ドロップゾーンDoroppu Zōn) is a zone in which a used card is put into, and cannot be used again.

In official playing field, it is located to the player's right, below the player's Deck Zone.

Way to put cards in Drop Zone
There are several ways for cards to be put in drop zone. The following actions will always put card into drop zone, unless stated otherwise.
 * Retiring Rear-guards and Guardians.
 * Caling rear-guard onto a Rear-guard Circle with a unit. The already existing unit will be retired.
 * Healing cards in Damage Zone.
 * Cost or Effect that state "put card(s) in Drop Zone".
 * Soul Blast