What is the meaning of ref number?

Excuse me, could you please clarify what exactly is meant by the term "ref number"? Is it a unique identifier used to track a specific transaction or an order? Perhaps it's a code assigned by a financial institution or a cryptocurrency exchange for internal purposes? I'm a bit confused about its significance and how it's utilized in the world of finance and cryptocurrency. Could you elaborate on its meaning and provide some context on when and why it's used?