Memo / destination tag
An extra reference number some blockchains use alongside the address, which shared exchange deposit addresses need in order to credit the ri
On some blockchains, an address alone is not the whole delivery instruction. XRP calls the extra field a destination tag, Stellar calls it a memo, and several other networks have equivalents: a short reference that travels with the payment.
The field exists because large platforms often receive deposits at one shared address per asset rather than one address per customer. The tag is how they know which customer to credit. Send XRP to an exchange with the right address but a missing or wrong tag and the transaction succeeds on-chain while the exchange has no idea whose it is. The money is not lost, but recovering it means a support ticket, proof of the transaction, and a wait.
The rules of thumb are short. When a deposit screen shows both an address and a tag, both are required, and the wallet you send from will have a field for each. Transfers to your own self-custody wallet generally need no tag, because the address already identifies you. And when in doubt, send a small test amount first, confirm it credits, then send the rest.
It is one of the few places in crypto where copying the wallet address perfectly is still not quite enough.