This add-on will add an extra permission to a group per group basis for users if they can reply to threads started by other or not. By default it’s set to No. You can set the restriction on a forum by forum basis as well by going to Node Permissions.
By doing this, some user groups will have more permissions than others. Because just only them could reply in a thread which is assigned just for them, instead of others.
And in general, the group without permission to access must purchase the right to do it and upgrade their titles in forum.