A Tag or Memo is an attribute added to a transaction.

Sending a cryptocurrency requires several things. First, of course, the wallet address. This address can be compared to a bank account number. One address cannot be used for different currencies. Clients of an exchange or broker sometimes receive a personal address for Bitcoin, for example. In this case, an additional attribute is not necessary. However, what often happens is that the broker or exchange uses an address that contains the funds of multiple clients. If a customer wants to send Bitcoin from one exchange to another, an additional attribute must be added to the transaction so that the exchange knows which customer the funds should go to.