It is its own community that happens to be federated with Lemmy and Mastodon. You can toggle federation on and off, and when off, you only see kbin's material. Otherwise it functionally acts like another lemmy instance, with a different interface and features missing from base lemmy.
There is also a microblog attached to it that connects with Mastodon in the same way. Each community has its own microblog that acts as a hashtag when on Mastodon.