The Su-30SM2 radar can only gimbal up or down 60 degrees, but when in TWS mode these limits are stabilized relative to the surface of the earth, while in track PD mode they are relative to the airframe.
The result is that TWS mode can find targets that the radar can’t physically look at in track mode, and PD mode can track targets that TWS can’t see, depending on the airframe position etc.
Please watch the attached video, where I am able to find a target on TWS but then when I try to lock it, my radar can’t turn far enough, and as soon as the radar lock is lost the target shows back up on TWS.
This is because, in the video, the target is ‘below’ the airframe in the airframe’s reference system, but ‘to the side’ of the airframe from the reference frame of the earth.