What is the _UID used for in Cappuccino?

Cappuccino defines a _UID property for every instance 开发者_如何学JAVAof objects it creates, but is this used for anything?

It's used in dozens of places, which you could see by doing something like "git grep UID" on the source code. Most often it is used as a hash key. Also, it's spelled "Cappuccino."