This is a full 3D manipulator.
The base rotates about the vertical axis
The first arm rotates about the local x axis of the first link
The second arm rotates about the local x axis of the second link
The motion of thered ball is tracked
In this problem, the point of interest -- the red ball is not a "computer graphics child" of the second arm.
Rather, it is a free object: its position is computed from Equation Q in Problem 12.3
A black vector from the inertial frame locates the position of the point of interet.
All frame connections are shown by the combined (frame, location) notation.
Finally, a blue vector at the tip indicates the velocity vector for the red ball. This vector is not a graphics object; rather it is computed using equations (P), (Q) and (R) of Sample Problem 12.6