Welcome to Sweet Home 3D. Hope you won't regret your dive in its source code. [;)]

I programmed the grade property to display some kind of grade associated to each piece of furniture that would help users to select one 3D model according to its popularity. At the moment, it's not used in Sweet Home 3D.

I would also like to know if modifying DefaultUserPreferences.properties, can only be done through recompiling the application?

You can modify this text file directly in its jar file if you want.