So I'm studying up on things to look for when finding a house to flip or wholesale. I read to break it down into 25 different categories, one of which is exterior concrete. Siding, porches, and Foundation are already its own categories, so what else can this exterior concrete be referring to? And what is it that I'm looking for? 

That is exactly what you are looking for. "Is there any major concrete and or foundation work I need to account for in my ARV." Concrete/hardscaping is paved walkways, driveway stairs ect. Foundation is exactly that, cracked stem all, cracked slab ect.

If you see a crack in the concrete/foundation document it.  Ask a professional and use common sense.

Obviously, a hairline crack in a walkway is not an issue.  However, a crack in a stem wall where you can fit a framing pencil in between may be a concern.