mastodon.xyz is one of the many independent Mastodon servers you can use to participate in the fediverse.
A Mastodon instance, open to everyone, but mainly English and French speaking.

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active users

JC Brand

@debacle @stevenroose @0

To be honest, I don't know. The configuration options come from the server. Sounds like this is a server feature where you can have a PubSub node associated with a MUC. But I don't know the particulars of this or which XEP it relates to.