In Virtualmin, a sub-server is essentially a fully separate virtual server (like a mini-hosting account) nested under a parent domain, allowing for unique administration, databases, email, and FTP, but sharing the parent’s Unix user/group for basic isolation, making it ideal for hosting subdomains or other domains that need independent management, distinct from simple DNS subdomains or aliases.
You create them by selecting “Sub-server” when using the “Create Virtual Server” function, specifying the parent domain, and then setting up the new sub-domain name (e.g., blog.example.com) as its own virtual server.