There are multiple "design surfaces" in VS. Depending on the context, it
could refers to Windows or Web Forms designers where you can layout the
controls in your app. it would also refer to the BizTalk designer where you
can orchestrate the various elements in your system visually. There is also
a visual surface for Windows and Web services where you can lay out data
controls, sources, and other objects. Throughout Visual Studio there are
many "design surfaces" that enable you to visually lay out the elements of
you app, reducing the plumbing code you have to write.