The reason that there are many classes identified are that there are many classes in common use. The need for iso, jis, jpn, prc, asme, and etc etc seems to make a simple task vary complex. I still prefer the definition of all sizes in their native dimensions, rather than a conversion to a common base. I like all the other suggestions from Portland, but this one does not appear correct. Seems the folks who write drivers feel more natural operating in one set of units.
Conversions are the realm of the application.
It certainly is simple. We used the same strategy of using only a single unit system in the IPP Production Printing Attributes - Set1 extension, instead of having both metric and English. We felt that was precise enough. The client should do some parsing and possible converting of units to the one that the user prefers. I have my problems with the new proposal.