Domains are an integral part of the middle-tier Relativity server that comes with Data Abstract. You can think of a Domain as a mini server, where each domain has its own logins, connections that define how to access databases, schemas that define what data is served from those databases and business rules to fine tune and control how the data is accessed and updated.
A single instance of Relativity can host multiple independent Domains, that are exposed to client applications as you define. Each of those domains are totally unrelated to each other, which means that a single server can be used to host services for totally different customers, projects or applications.
To learn how to create a Domain checkout the documentation for the tools that are supplied with Data Abstract