An Unsubscribed contact is someone that has been communicated with (or communication has been attempted), and has subsequently been revised to not communicate with. There are several possible reasons this may occur.
A Not subscribed contact is one that has never been communicated with in ActivePipe.
This may be due to having an invalid or missing email (previously categorised as "Bad email") or mobile number. In this case, if a contact's email or mobile number is corrected, their subscription will change to Subscribed.
This also applies to contacts that are already marked as unsubscribed when your ActivePipe account is first connected with your contact integration (usually when your ActivePipe account is set up).