balihr - I need to see what you have so far to fully understand how what you want can work. You can't select a subcat from the categories-tabs menu (unless you have a dropdown mod installed). How are you getting the subcats displayed in the first place to pick one? Do you want all of the subs of a top cat to display when you pick it, and only one subcat and its subs to display when you pick that? That leaves a situation where you can't easily move from one subcat to another in the same top cat.

bonboncat - Again, I need to see what you have to advise on how to finish getting your other links to behave as you want.