The arm and racket for the high level serve slice and flat serves have specific angles. Neither the arm or racket are close to vertical at impact. (The kick serve has different angles.)
Viewed from the side of the ball's trajectory-
1) the arm is tilted forward
2) the racket
appears vertical from this camera viewpoint and the palm of the hand on the racket handle also
appears vertical.
3) the wrist is extended to make this work.
Viewed from behind camera, looking along the ball's initial trajectory -
1) The arm appears to tilt to the right.
2) For slice and flat serves the racket appears to tilt to the left.
Thread showing racket and arm angles with pictures.
https://tt.tennis-warehouse.com/ind...-flat-slice-kick-impacts.489960/#post-8099755
Racket demo using these angles with a comfortable extended wrist-
Eastern Forehand Grip. If I hold the racket in the position of impact and use an Eastern Forehand grip (index knuckle on bevel #3 and handle butt at the little finger) the racket strings have an angle in the open/closed direction. Looks close to neutral between open or closed.
Continental Grip. If I hold the racket in the position of impact and use a Continental grip (index knuckle on bevel #2 and handle butt at the little finger) the racket face is more closed.
I guess experience has shown that with a high level serve the Continental or 'Strong Continental' is the preferred grip for the average high level serve.
For a Waiter's Tray serve technique, the most common technique used by recreation players, who knows what angles apply? The WT is associated with the Eastern Forehand grip. WT develops most pace by closing the racket without the additional arm and racket rotation from ISR. For the same pace, I believe that high-low errors are greater with a WT.