From Shadow Era Wiki

Unique is a category of cards within Shadow Era.

Unique cards are generally cards of exceptional strength. You may therefore only have one copy of any given Unique card on your side of the field at any given time, and may not play another copy of the same card until the first one is destroyed. Your opponent may have a copy of the same Unique card on their field while you have one on yours though. In Call of the Crystals, the only cards to be Unique were allies, however Dark Prophecies introduced attachments with this sub-type as well. Also, any Artifacts, Support Abilities, Weapons and Armors (but not Traps) are also considered to be Unique, although they don't have Unique printed anywhere on them.