The Universal Rarity Scale (or URS for short) is a 10-point scale used to label a coin or other collectible’s rarity based on how many examples of it are believed to exist, according to PCGS. 

The least rare collectibles according to the URS (items labeled UR1) are ones of which at least 10,000 examples are believed to exist. The most rare collectible (items labeled UR10) are unique, meaning only one example is believed to exist.