In relation to camber...
It depends also on how much roll the car has and how raked the corners are. If the car has lots of body roll, meaning the center of mass is displaced further from the center of the wheelbase, the "weight" of the car will push down on the tire at an angle. One of the jobs of negative camber is to angle the tire's contact patch so that the "weight" pushes more evenly on it and you get more grip around corners. Of course, more grippy tires means more cornering force, thus more body roll and the 'right' amount of camber increases.
But if the front of your car rolls a lot, and the rear does not roll much, and you try to introduce understeer by increasing the rear camber, the rear may get less grip because its no longer having its contact patch perpendicular to the direction of the force being applied to the tire. So the result can be skewed.
But that is on a flat course. If the corner is raked, depending on the angle of rake, that may mean that you get more grip by having more than "enough" camber.
Some cars also, like the Ford Mustang, have a live axle so that when one tire lifts, its camber is changed (the wheel moving up, say the outside, points inward, like and \ and the outside wheel mimics it so the wheels look like this: /---/ with the left wheel (imagine you are looking at the rear bumper of the car facing away from you) bumped up or this \---\ with the right wheel bumped up. Using camber can make it grip better so that when cornering at an angle, the tires look like this /---| and |----\. This will give the rear more grip, and thus introduce understeer compared to whatever the setup had before you adjusted the camber.
Make sense?