When a User/Subscriber is subscribed to a Newsletter, he will automatically receive all new Issues of that Newsletter. Each Issue is an email Campaign that is specifically sent to the Subscribers of a given Newsletter.
Subscribers can unsubscribe at any time. Each email they receive contains an unsubscribe link. They can also manage their Subscriptions by editing their User profile.
Note: b2evolution 6.7 manages subscribing to a single newsletter. In the future il will be possible to subscribe/unsubscribe to multiple newsletters.