Dynamis (神域 or デュナミス Dyunamisu) is the place where Z/X Extra and the upper part of Z/X Overboost cards are placed. It is a separate deck from the player's own deck. In the story, it is the name of the place where the Dingir lived.
Player may put up to 10 Z/X Extra and up to 4 upper part of Z/X Overboost cards in his or her own Dynamis for a total of 14 cards. The rule that states that you may only have up to 4 cards with the same name in a deck still applies to Dynamis. During a game, Dynamis is put into the Dynamis Zone in face-down position. However, a player is allowed to look at the contents of his or her own Dynamis anytime during the game.
Z/X Encyclopedia Entry
"Dynamis" or "Dunamis" (δύναμις) is the Greek word for "power", "strength", or "ability". The base kanji for Dynamis (神域 Shin'iki) means "sacred area" or "sanctuary", a reference to Dynamis being the living place of the Dingir as well as place for Z/X Extra cards in general.
- Z/X Encyclopedia (Japanese)