Bip39 Checksum. The passphrase is not really chosen from the predefined list of 2048 words – it could be any word or phrase you choose. At its core, a BIP39 passphrase modifies the cryptographic strategy of wallet generation. In this article’s a technical have a look at how it works: https://travialist.com/story9890150/bip39-passphrase-https-linktr-ee-bip39-a-simple-key-for-bip39-unveiled