The protocol allows a service to choose what type of channel they want to create. This enables several other business use cases than just delivering information via notification.
The default channel, this channel is created by the service and is intended to be open for any user to come and subscribe without any restrictions. Channel can also indirectly subscribe the user to it by paying the user a minor fee.
A service can opt to create a closed channel, this channel cannot be directly subscribed by the user. Instead, the channel needs to add the user indirectly by paying a minor fee to the user.
A service can opt to create a mutual channel, which requires user direct action to subscribe to it, but the subscription is only confirmed once the channel approves it as well.