I just meant different servers need different subdomains?
It doesn't matter to the end user on another server, because federated posts will all look similar even when they are from totally different server types. BookWyrm and PeerTube posts will look similar if you're following them from Mastodon etc.
But it may matter to the users who are logging into these different servers, because they will need a URL to log in at. If the subdomains are too vague and similar, users may mix up which one is which.
It's not a big issue though, mainly an aesthetic one :)