In another thread
Peter Constable said,
A bit of feedback on this font -- which also applies to Amstelvar: these are using custom axes, but are not following the spec requirements for custom axis tags:
Like other OpenType tags, axis tags are four unsigned bytes that can equivalently be interpreted as a string of four ASCII characters. Axis tags must begin with a letter (0x41 to 0x5A, 0x61 to 0x7A) and must use only letters, digits (0x30 to 0x39) or space (0x20). Space characters must only occur as trailing characters in tags that have fewer than four letters or digits.
Privately-defined axis tags must begin with an uppercase letter (0x41 to 0x5A), and must use only uppercase letters or digits. Registered axis tags must not use that pattern, but can be any other valid pattern.
has details on axis tag requirements.
I am curious about the part I have marked in bold above.
I can see that "Privately-defined" axis tags can only use uppercase letters or digits in order to differentiate them from "Registered axis" tags at-a-glance.
But this makes me concerned about what kinds of privileges "registered" axes have over what were, last time I looked, called "custom" axes?