Scale in Type Network parametric axes

[PeterCon]

The descriptions for all 11 axes in the Type Network proposal all specify the following for scale:

Scale interpretation: Values should be interpreted as per-mille-of-em.

That defines the units of measure, but they don't fully define a scale because they don't specify what is actually being measured. It could refer to anything for which per-mille-of-em could possibly make sense; e.g., the distance across the widest opening of ink traps in glyph 72.

The details for each axis should specify not just the scale but also give some indication as to what is assumed to be measured.