[Yoda]

>If the context is in doubt, how do I decide which of these two
>meanings is appropriate?

The context should always make it clear whether the word is being used
in an absolute sense, or relatively. If no relative is specified, take
it as absolute, meaning vertical or upright.

--

Weightshift,

I applaud your research and the intellectual curiosity that inspired it. You are on the right track here.

In TGM, as a Basic Plane of Motion, the term Vertical is used in an absolute sense. As a geometrical alignment, it is used in a relative sense, i.e., defining a right angled relationship to any one of the Three Basic Planes -- Horizontal, Angled or Vertical.