This is from the Business Object Mapping section of the Learning Resources:
When an EDMX file is saved for the first time a new project item will be added to the project containing the EDMX file. This project item will have the same name as the EDMX file but with a further ‘.tt’ extension. Several additional files will be generated ‘under’ this project item. One of these will be a ‘ReadMe’ file that explains in more detail how to customize template generation. The other files will have an ‘.IB.Designer’ extension and will contain the DevForce generated code. Feel free to inspect these files but do not modify them because they will be regenerated each time the EDMX file changes. In most cases there will only be a single such file, but there are options described later to allow the generated code to span multiple files.
One further side effect of this operation is to ‘turn off’ the standard Entity Framework code generation mechanism. DevForce does this by modifying the ‘Custom Tool’ property associated with the ‘EDMX’ file by adding a ‘<RemoveToRestore>’ prefix to the custom tool name specified there. (At any time this prefix may be removed to restore the standard Entity Framework code generation. It is usually a good idea to remove the DevForce ‘.tt’ file, and its underlying children, after this is done to avoid conflicts.)