As noted in the Introduction, Data Abstract provides two types of servers that you can use as the middle tier of your project:
Relativity Server is available with all editions of Data Abstract and enables developers to deploy a Data Abstract server without them needing to create or maintain their own server application. It can be deployed to Windows, Linux and Mac OS X and is also capable of being deployed to cloud hosting services like Amazon Web Services or Microsoft's Azure platform.
If you are using Data Abstract for .NET or Data Abstract for Delphi then you can build a custom server that is tailored to fit your exact needs with custom services & business logic. You can then build your schemas, business rules scripts and connection definition into the executable which simplifies deployment to your customers as they won't need to install Relativity Server and configure it.
For a more in depth look at the difference between the two server types, and why you might choose one over the other, we recommend reading the "Choosing between Relativity Server and a Custom Server" article.