The tmdl file (short for test model file) contains the data for the method you want to test. It includes input factors, equivalence classes, representatives and test cases with all it’s values and properties.
Edit it in the user interface of devmate in you development environment (e.g. Visual Studio or IntelliJ).
Avoid manual editing of the .tmdl file because the file structure may be broken and devmate will will no longer work correctly!