For most applications, you can leave this blank. If you do not set up any user levels, PonG will assume your application does not have different user levels.
If your application has multiple user levels you can set them up here to set up the preferences for different user levels. Here you can set up a different configuration prefix for each level. Then you can set up in the dialog which widgets appear in which user levels. The levels are not "sorted", they are just different strings, so it is not relevant which is first or second.