Purpose of Edi project
Edi project file (.csproj) is a bridge between a data document (.xlsx; .xls) and a text document (.docx; .doc; .odt). Edi produces a serial letter linking data from the first document with text from the latter one.
The project file contains common project information, e.g. storage location and settings of the latest mail merge into multiple files.
Update data source automatically
After replacing data document with another one, the data source is automatically updated next time the project is open. Created serial letters have the same layout, but different data content.
Use different Serial Letter Layouts
On the other hand, if you replace the text layout document, you can present your data in a different way.
Of course you can replace both documents at the same time.