Using fractions does not allow you to sort by the SubSeries. Or on a Sony, create another collection based on the Sub Series.
Setting a second custom column for Sub Series allows both of these.
I use fractions for books or stories that are published to fit between the main books in a series. The Stephanie Plum books have books inserted in the main series. IIRC there are a couple of Short Stoires inserted in the Harry Bosch series.