I recently asked the following K2 question via Twitter:

I guess I was hoping that this functionality was something that I'd missed in the module configuration somewhere; seems like a pretty obvious requirement, especially if you have parent categories setup for very different purposes, each with their own set of sub-categories (like, a news section or a blog with several sub-category subjects).
It stands to reason that such a requirement would be quite necessary, but when I didn't get an answer to my Tweet, it dawned on me that this functionality was overlooked (or perhaps not thought of) by the K2 developers.
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