Thank you for the link. It describes the functionality from the user perspective.
But I have more technical question - it’s about extending email entity with extra field (for example, text input).
It will be unique for each email template.
You can store the information within a var in each e-mail template or you can extend the email entity and access the var with a subscriber (or autoload in the extension).
Maybe you should just add a custom field ? Anyway I think the settings dialog in the backend will not work here and you have to add the field programmatically.