A crypto withdrawal is a transaction that allows you to move your cryptocurrency balance off the Revolut platform to an external crypto wallet that you control such as a Ledger or a Trezor.
This is an 'on-chain' transaction, meaning that your crypto is transferred via the blockchain. This is different to a P2P transfer to another Revolut user (also known as an 'off-chain' transfer), where Revolut updates its internal ledger to reflect the transfer, but no actual transaction is made on the blockchain.
In order to transfer crypto to an external wallet, select the BTC screen or on the 'Crypto' section of the Home screen, tap on '...' then 'Withdraw'. There, you can add an address using the QR code or by typing it in manually.
Crypto withdrawals are not reversible once executed.