If you want to make encrypted digital backups
Why would you want that? The title of this thread has to do with securing your seed phrase. Backing up your wallet file is, first and foremost, another topic, and secondly, worse comparably to simply sticking with the seed phrase.
Yes, you're right. Although I knew this detail, I didn't make it very clear. I think extended words is the best definition.
Just to avoid any potential confusion.
- BIP39 passphrase does not encrypt, it is an extension of the seed phrase. All BIP39 seed phrases have an empty ("") passphrase by default, and you can choose to extend that.
- Wallet passwords are used to encrypt wallet files (such as Electrum's password), but as I already said, that's going off-topic.