An item is Bind on Use (BoU) if it becomes Soulbound when first used. This is used for objects that are used but do not need to be equiped, such as a mount. A warning appears before a player uses a Bind on Use item for the first time, asking if the player is sure they want to do so.
Vendor-purchased mounts, small pets, and other items can be BoU.
Other loot may be called Bind on Equip (BoE) or Bind on Pickup (BoP).