[Loren]

It is a specific Basic Plane Angle, regardless of how the
swing is powered.

Paraphrasing from the book in 10-6-0:

Plane angles are classified on the basis of reference points.
One of the reference points is that point reached by the
right shoulder after a Flat Backstroke Shoulder Turn.

The right shoulder and hands are always precisely AT the
right shoulder plane at "The Top" regardless of plane angle
or shoulder turn. Any other shoulder turn can also provide
the reference point for a Turned Shoulder Plane.

This plane angle has far better performance characteristics
than any other because any plane angle shift is hazardous.

This does not refer to the Shoulder Turn Takeaway which is
always too flat and/or too low, making a plane angle shift
mandatory.