The "acceptance angle", as I understand it, refers to the orientation of the antenna that produces the strongest signal, give or take some level / db. It is sort of inverse to the "rejection angle", where the antenna produces the weakest signal or null. Ideally, a directional antenna (such as a Yagi) will have a fairly narrow angle of acceptance, so it delivers a lot of clean signal and very little spurious noise or distortion. On the other hand, an omni-directional antenna will receive signal in a broad area, even up to 360 degrees. If you are using a directional antenna, it makes good sense to mount it with a rotor. That way, assuming your tuner has a good signal meter, you can aim the antenna directly at the strongest signals in your reception area.
