The users registry is maintained in the protocol to carry out various tasks such as stopping channels from adding them again (after a user has unsubscribed), if their public key is in the registry, what channels they are subscribed to and a mapping to get to those channels, if they own a channel, etc.
If the user is already in the ecosystem
If their public key is already in the registry
Whether they own a channel
The channels they have subscribed to
The channels they have graylisted