The point of it...
Set columns are a bit different from most columns of data because often you will have marked only some members of the Set column. Accordingly, we need to sort so that
othe entries which appear in the Set column come at the top
osorted appropriately
oand the remaining entries get sorted in some default way.
Header sort for Set column
This key word list has Set column entries.
After sorting by clicking the Set column header (purple arrow), we get
The remaining entries are sorted alphabetically.