Bip39 Passphrase. The seed phrase could be transformed to some range which happens to be employed as being the seed integer to the deterministic wallet that generates each of the key pairs used in the wallet. For words that only have three letters, there isn't any 4th letter. One example https://directorystumble.com/listings545707/bip39-mnemonic-https-linktr-ee-bip39-bip39-word-list-no-further-a-mystery