Bip39 Seed. The passphrase just isn't picked with the predefined list of 2048 words – it might be any word or phrase you select. BIP39 is usually a design implementation that lays out how copyright wallets make the set of words (or "mnemonic codes") that make up a mnemonic sentence, https://directmysocial.com/story4263958/bip-39-https-linktr-ee-bip39-helping-the-others-realize-the-advantages-of-bip39