Additional registered axis tags can be added in future versions of the specification. The working group decided to wait to see what kinds of axes emerge, rather than trying to second-guess type designers and perhaps register a lot of axes that end up not being used. If something like x-height ratio becomes a common axis in fonts, then it would make sense to register it.

The first five registered tags all correspond to registered attributes in the new STAT table. This is important, and explains why those particular five axis tags were defined. I am considering writing a follow-up article about axis tags and STAT attributes, since the existence of an ‘ital’ axis is already confusing some people who wonder if it will be normal to roll roman and italic designs into a single variable font (it won’t).