At the moment there is only one level of sub category. Let's say you have a main category of shirts, you could have sub categories underneath that of say mens shirts, women's shirts, but that is all that exists at present within the current pro version.
I don't know the complexity of putting more levels of sub category within later versions of pro, or what sort of timescales there would be on that. Only one of the programmers from the development team could give you a definitive answer on that point.
Now that it's Nov. any word on more levels of sub-categories.
Thanks