I can visualize this very clearly in my mind but I may need some graphics to illustrate what I am trying say but I am not interested in spending lots of time on that. Sorry about that.
Let me try this way. The linear angle (such as for thow angle) generally is between two straight lines (linear) such as betwen two sides of a triangle but the angle values for trajectory curves such as parabola or hyperbola etc is little more involved but is not the same linear angle between two staright lines meeting each other. The throw angle is the angle between a horisontal line and another line pointed upwards for top spin (or downwards for backspin) or sidewards for side spin. The horizontal line is the ball coming towards the racket and the other line is the line pointing upwards (for top spin) & downwards for backspin. This measurement takes place just after the ball meets & leaves the racket at an angle , the throw angle.
Of course teh ball very rarely comes towars a player in a perfect hoeizontal ine. And very rarely is the racket hel by a player in a perfect verical position. These are for measurement purposes only
The trajectory curve is a depiction of the ball travel from the instant the ball leaves the racket to the time when it lands on receiver's side. The trajectory angle mesaurement is a measurement of this entire curve but I am not sure if it is important from a practical standpoint except that a more spinny ball with lesser speed & will have a more curvy trajectory such as coming from a mushy all wood slow blade as compared to a flatter trajectory faster less spinny ball coming from a stiff composite blade. if you are more intereste in
Curves and angles between them. just google for it.