The legacy single-axis representation of an email's audience and
archive visibility. No longer stored: filters owns the audience
axis and archival_mode owns the archive axis, and the deprecated
API field is derived from those (see email_type below).
Emails that are only available in the archive and not sent to subscribers.
Emails sent specifically to subscribers who were previously premium but have since churned.
Free emails are sent out only to subscribers who are not paying for your newsletter (so you can send specific emails to convince them to pay, for instance!)
Premium emails are sent out to only paying subscribers (including those with gift subscriptions or on a free trial), and only premium subscribers can view them in online archives.
Private emails are sent out to all of your subscribers but are not viewable in your web archives.
Public emails are sent out to all of your subscribers and are available in your web archives.