I am not laylah (obviously) but I read your "theme" and "category" definitions as right.
...Subject is odder. For: - You may make two claims at a time, should you so desire; they can be for the same subject or two different ones. I read subject = character/pairing/grouping/fandom
While for: - You may not claim a subject that someone else has actively claimed; you MAY claim a subject that someone else has finished or dropped. I read it as the whole chunk you're claiming - character/pairing/grouping/fandom + chosen category or 'mixed-bag.' (and possibly also medium, but that's not mentioned.)
And therefore only one person can claim Kimberly/Archer for any specific thing at once, whether it be an actual category or mixed-bag.
NOT THAT I HAVE ANYTHING TO DO WITH THIS CHALLENGE. IN FACT I didn't even notice it or consider it because I have so little free time as it is that it would be silly to even look at it! >->