Every subscriber can have a tracking tag assigned to their subscription record.
If a subscriber signed up to your autoresponder through a form, their tracking tag will be the value of the “tracking_tag” field on the form. The “tracking_tag” field would normally be a hidden field.
If a subscriber sins up by email, the subject line of their email can contain a tracking tag. For example, if you wanted to accept subscriptions by email, and to track the source of those subscribers, you could use a link like this:
mailto:info@mydomain.com?subject=TRAad56
The subject line will contain TRAad56, and the tracking tag “ad56” will be assigned to the subscriber.