A LimitedU (or Limited Unique) item is a hat, gear, or face that has only a certain amount in stock. Each copy of the item is stamped with a unique serial number (e.g. #412/1000). The item will only be able to be bought a given amount of times, until it is sold out. After it has sold out, it can only be obtained from other owners of it. Builder's Club and up members are the only members allowed to sell or trade these items, meaning they cannot be sold by players with free accounts.
Limited Uniques become increasingly rare, as there is a finite amount of them in circulation, and the amount of them can only decrease, by being lost or deleted. They are commonly sold by dealers for a price over what they were originally sold for in the Catalog. These hats can only be bought and sold for ROBUX.