You can register your username, if you wanted to keep it, on another server as a backup as well

Well, yes and no. For instance (no pun intended), the site for your home instance is a page you’re going to be landing on a lot, so you’d want one which loads smoothly and reliably for you, and this often depends on your instance’s server infrastructure. Some larger instances are already being overloaded. Another concern is probably stability and longevity, since the fate of your account is tied to the instance that hosts it, so you’d have to choose one you trust. And the last one is moderation: you don’t want to join an instance where the admin is known for being shady and randomly banning people or blocking lots of instances. That being said, so long you choose a reliable instance it doesn’t matter too much which one it is.

Instances can de-federate each other so you can’t see communities. Beehaw just de-federated us due to the large influx of users containing some people trolling their communities. The moderation tools are not great right now and so rather than dealing with the influx of trolls they de-federated this instance.

So you can’t see Beehaw communities and their users cannot see your posts if you post on another instance’s community.

This lets instances decide who they want to associate with and is a good thing for the long-term health of social media. You don’t want to be a part of instances that associate with terrorist instances or other shady things, de-federation is the tool to keep that from happening.

In general, any instance is as good as any other but as time goes on there may be a few different ‘islands’ of instances that are functionally separate from each other and you’ll either choose one or use multiple accounts to swap between them.