It's pretty obvious what they do if you try:
A separator is not a category, just a visual separator.
Though if you click on it, it will show all categories below it combined (until next separator or the end)
Lets say you wanted to very neatly organize Game mods:
Tixati's general status categories, Seeding/Downloading/Queued etc.
If you clicked on GameMods separator, you'll see ALL items for categories 2,3,4 together. If you click on GameTrainers, you'll only see one category with trainers because it's the end of the list.
Few notes: Apparently you cannot add the separator as a whole to the Shares tab or copy the magnet-links although the menu is clickable.