I actually don't know what the correct technical name for the offtopic ('Finally...') subsection of the forum, but sub seem like a convenient shorthand for any hierarchy system (forums, message boards, folders, domains, etc), especially now days when new platforms often utilize mish mash of elements often under new names when was the last time you heard the term 'message board' in the mainstream..
Interesting, I had never considered the correct technical name for such places, I didn't even know if there was one for stuff on the level of "finally...". Now that you bring it up, I went and checked SMF's manual, it refers to sections on the level of "Finally..." as "Categories". And of course Boards for the level of "General Discussion" or "Creative Projects" or the like, and Child Boards for things like "Roll To Dodge"
I don't know if I've ever heard of people refer to them as categories, although perhaps that is because most people don't tend to talk about categories together anyway. I think most of the time when people talk about such things they say "Upper boards" to refer to the "Dwarf Fortress" category (And announcements) and "Lower boards" to refer to the Finally...+Other Games. Since there tends to be a bit of split in the community between these two sections (although plenty of people that use both of course, and plenty of people that don't use all the same lower boards)
Ultimately doesn't matter of course so long as we're mutually intelligible.