This plugin adds a per-category setting allowing staff to restrict replies in a certain category to the OP, and members of specific groups. This is a bolt-on to the core permissions system, and is configured using a separate UI underneath the regular category security settings.
You may want to use this to create a support category where all users can ask questions, but only users in the "experts" group can post replies.
For more information, please see: https://meta.discourse.org/t/restricted-replies-only-allow-certain-groups-to-reply-in-a-category/131343