Thinking of attitude control in terms of axes is a bit complicated, however.
Pitch is when the nose of an aircraft moves up or down.
Yaw is when the nose moves from side to side. Roll is when the wings dip to one side or the other. Although we usually do not talk about satellites in terms of nose, tail, and wings, the principles are the same. Betaprime is positive when the sun is above the orbit plane and negative when it is below. Proper orientations of the satellite's altimeter antenna and solar array are governed by the value of betaprime.