Bip39 Checksum. The passphrase is just not picked within the predefined list of 2048 words – it could be any word or phrase you select. What constitutes a "very good wallet" is one which has distinct, very easy to obtain documentation on supported BIP standards, derivation paths, and recovery processes. https://extrabookmarking.com/story18700093/bip39-https-linktr-ee-bip39-getting-my-ian-coleman-bip39-to-work