Bip39 spec https://files.fm/u/2eemhhak2q#design Despite the fact that it takes advantage of BIP32 system you are able to still import BIP39 seeds into Electrum wallet. Also aside from BIp32 There's BIP44 Edition which some wallet use for that generation of seeds. Just how much security does a passphrase increase? A BIP39 https://ztndz.com/story25777809/bip39-generator-offline-a-secret-weapon-for-bip39-mnemonic-code-converter-tool-https-linktr-ee-bip39