Bip39. The passphrase isn't picked from the predefined list of 2048 words – it can be any word or phrase you select. BIP39 is usually a design and style implementation that lays out how copyright wallets produce the set of words (or "mnemonic codes") that make up a mnemonic sentence, https://bookmarkick.com/story19111248/bip-39-https-linktr-ee-bip39-not-known-details-about-bip39-wallets