Bip39. The passphrase will not be selected with the predefined list of 2048 words – it may be any word or phrase you end up picking. BIP39 is a layout implementation that lays out how copyright wallets build the list of words (or "mnemonic codes") which make up a mnemonic https://bookmarkuse.com/story18882822/bip39-passphrase-https-linktr-ee-bip39-about-ian-coleman-bip39