Arbital domain is a way to group pages together by category. Mathematics, Arbital, and AI alignment are all examples of Arbital domains. A page can potentially belong to multiple domains, and a page that's not in any domain is considered unlisted.
You must be trusted in a domain in order to create pages there. If you are not, you can still submit your pages for approval. Once you've made some contributions to Arbital, you will be promoted to trusted.
Preserving quality of content
Every author is an expert in different domains. Just because someone is an expert in physics, doesn't necessarily mean they are an expert in psychology. For this reason, a user has a separate trust level for each domain.
Pages inside a domain have likely been reviewed by relevant experts and likely doesn't contain any major errors. (It doesn't mean they are complete, though.) Unlisted pages don't have any such assurance.
How is a domain organized?
When a domain is created, one page is designated as the root of the domain, e.g. math domain root page. Any descendant of the root page automatically belongs to the domain.