Every email subscriber has a source and status associated with it.
Status is simply subscribed or unsubscribed. When a subscriber unsubscribes from your newsletter, we are required to store them as unsubscribed, to prevent them from being accidentally resubscribed.
There are 3 possible sources: Signed Up, Imported or Manually Added
Signed Up is someone who has added themselves to your newsletter
Imported is someone who has been added through CSV Import
Manually Added is someone who has been added manually on behalf of a collaborator of a publication