Basically you can use the "backup" partition for anything you want. Does it actualkly hace anything in it know?.
It would make more sense to have your partitions arranged as c=boot up, basic operating system and programs you want/need.
d= all the data you generate e.g. Photo's, documents, etc so that when you want to do a back up just burnm the files you want onto a cd from here.
e= recovery partition with operating system on it so if you ever accidentlt delete any OS files then can coppy them over from here, also could boot from it if required.