Sensitive Categories has things like dating, politics, religion, etc. Whereas General Categories has everything else.
Both of these sections show a list of possible Adsense categories along with what percentage of the ads you've displayed came from each category, and how much of your Adsense revenue came from each category. Most of the categories also break down into multiple sub-categories.
As soon as I found it I immediately realized I had a couple of categories with bad impression/revenue relationships. For instance I had one category that was accounting for 13% of impressions, however it only accounted for 3.8% of revenue. In comparison there are other categories that account for 6.9% of revenue on 3.3% of impressions and 4.5% on revenue on 2.5% of impressions.
So, I went ahead and blocked the categories with horrible impression to revenue relationships. I figure that having more ads from more profitable categories being displayed will translate into more money. Since Adsense has much improved reporting in V3 it will be easy to see just how well my changes do in the future.
There is a 50 category limit on the number of Adsense categories that can be blocked.