This plugin adds a per-category setting allowing staff to restrict replies 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.
In the regular Discourse permissions system, this might be called
Create / Reply (creator only) / View
You may want to use this to create a support category where all users can ask questions, but only users in the “Employees” group can post replies.
This plugin has a very specific use case. In general, we recommend keeping support categories open, so that the community can help each other answer questions.