Subscriber is any user that subscribes to one or more channel. A channel can also be a subscriber of other channels. Furthermore, a channel cannot send a notification to a user who is not a subscriber of their channel to ensure user is in complete control of what channels can or cannot send them notification.
Though means to delegate subscribe by the channel is given with greater incentives for the user (subscriber) to ensure win-win scenario  and to cover the need of the channel as well.
The channel owner will always be the first subscriber of the channel as soon as the channel is activated, it's not possible to remove the channel owner (subscriber) from their own channel.