Using the modeler, you can either design your model using surfaces or parts. A surface is basically a canvas on which you can CodeFluent Entities concepts such as namepsaces, entities, enumerations etc. On the other hand, a part is a XML file, and offers the same experience as developers know using the builder (CodeFluent.Build.exe) in command line with XML files.
However, both techniques are actually two views of the same in-memory model, and so both are in fact synchronized. Adding a concept in the part is reflected on the surface and vice-versa. This way you get the best of the two worlds: a fast, convenient and intuitive UI as well as an unbounded, human-editable, declarative view for power-users.