Size, Rule #1, and parallelism 2

[Burunduk]

A lot has been said about how a size tolerance with Rule #1 limits the form of a regular feature of size. It is not mentioned in the Y14.5 and related standards or other sources I came by (except a couple of threads in this forum) that, unless it is an oversight, Rule #1 also limits the parallelism error of each face of a width relative to the opposing face. Unlike for example, for flatness applied to a surface, the standard does not instruct us to always specify a parallelism tolerance smaller than the size tolerance associated with the FOS the surface is part of (of course I mean cases when the datum feature is one face of the width and the controlled surface is the other).
Does anyone have any assumptions why the indirect parallelism control by Rule #1 so rarely gets any attention, and why there is no corresponding guidance on non-redundant parallelism tolerance values?

 

[Nescius]

It's worth the few minutes to sketch out the situation. Some trends shine through if you play with it.

Roughly, this quasi-parallelism error varies from 1x to 2x the size tolerance as you move the crease from the end toward the middle. If you "go past the middle" so to speak, your quasi-datum becomes the small face of the tent, and that's weird. I chose arbitrary geometry for my sketch, but did keep the crease well away from the middle (and the end) to show the phenomenon with less possible distraction.

You may also note that if the inside radius of curvature is equal to the size tolerance and the crease is centered, the quasi-parallelism error is exactly 2x the size tolerance. This seems a bit more intuitive when you realize the center point of the inside radius is exactly on the rule #1 envelope boundary when the inside radius of curvature is equal to the size tolerance, whether the crease is centered or not.