What does 'Exposure' Mean?

According to Stovich on the Steam forums, the more enemies you expose yourself to, the more points you will get for each kill. Namely by alerting them to your presence and have them charge.

He provides an example:

You can plainly see this in the first room of Chapter 16, do one test run where you kill them without them noticing you, you will get +400 points per kill. Now expose yourself and let them charge at you, then kill them. You will net +800 points per kill instead.

Ishanji on the GameSpot forums chimes in on Severe Exposure:

Severe Exposure seems to be (not 100% sure on this) hanging out in the open with a bunch of guys coming at you or shooting. You'll probably end up getting a lot of these just because you have to play fairly ballsy to get the best combos, but if you can get more of them then your score will improve.