Also electrum have a custom seed whereas you can add different words of your choice and actually I used that option.
That's basically the same as a BIP39 "passphrase"... While people often describe the BIP39 passphrase as a "25th word" (from Trezor/Ledger 24 word seeds), the passphrase can be anything you like. A word, several custom words, a long complicated password consisting of symbols, numbers and letters etc.
It's the same with the Electrum "extend seed with custom words" option. You don't have to use words...
