Clarify input vs output in location units?

[tiroj]

I suspect this may be confusing to some font developers

u indicates the number is expressed in "user" units—the "input" values in a variable font's designspace axis map. d indicates the number is exprssed in "design" units—the "output" values in a variable font's designspace axis map.

because ‘input’ and ‘output’ depend on perspective. From a font maker perspective, the design units are in the input from the font and the user units are the output to the client software. From a user perspective, the user units are the input from the client software and the design units are the output from the font.

I think the latter is what is implied by the use of input and output in the variable location spec, but maybe clarify this in some way?

[skef]

Ah, that part of the discussion was meant literally and refers to designspace file map elements:

https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#map-element

So yes, abstractly, one could consider the mapping (and probably many mappings) to run in either direction, those terms refer to specific XML properties that aren't ambiguous in that way.