Bip39 Checksum. At the start you should store it SEPARATELY from your seed. Having your seed plus your passphrase in the exact same location defeats the goal of having a passphrase to start with. BIP39 specifies a list of 2048 words from which the mnemonic phrase is selected. Just about https://webookmarks.com/story4713015/bip39-list-https-linktr-ee-bip39-top-guidelines-of-bip39-words