I know this was talked about recently. I see the opt in button in my account section. That would opt in all my images. Is there a way to then go opt out certain images, or is it an all in, all out situation?

Thanks Luis, I didnt see any way to opt out individuals either but sometimes stuff like that is difficult to find.

Does anyone know what kind of images and for what use are licensed for sensitive use?

You're right, this info is not easy to find.
Prohibited under a standard or enhanced licence:
"Use of an Image together with pornographic, defamatory, or otherwise unlawful or immoral content or in such a manner that it infringes upon any third party's trademark or intellectual property    
Use an Image in a way that places any person depicted in the Image in a bad light or in a way that they may find offensive - this includes, but is not limited to the use of Images: a) in pornography, "adult videos" or the like; b) in ads for tobacco products; c) in ads or promotional materials for adult entertainment clubs or similar venues, or for escort, dating or similar services; d) in connection with political endorsements; e) in advertisements or promotional materials for pharmaceutical or healthcare, herbal or medical products, including, but not limited to dietary supplements, digestive aids, herbal supplements, personal hygiene or birth control products; and f) uses that are defamatory, or contain otherwise unlawful, offensive or immoral content. You may not use an Image containing the likeness of a person if such use implies that the model engages in any immoral or illegal activity or suffers from a physical or mental infirmity, ailment or condition."
 .... but it wasn't obvious or easily discoverable about under which circumstances sensitive use files maybe be purchased and used.

That's very interesting. These restrictions are more clearly defined and more widely ranging than iStock's. More likely to be acceptable by potential models, IME.