Hi Bill,
Yes, the 'Export' is incremental, so it checks if the artist/album is already in the 'Export' and if not then it copies that folder set to the 'Export' set.
One thing to be aware of is that if you are renaming artists and/ or albums, if that folder set has already been saved under its original name, then the next 'Export' run will save the renamed folders and you will have two versions of the same folders in the 'Export'.
The way to manage that is to begin a 'clean Export' every so often, so that you maintain a valid copy of your music collection.
Regards,
Peter.