Smith,
I did some digging and found this original MSN1 discussion in the archives:
https://www.pwg.org/archives/ipp/2001/010090.html
In short, there was a desire to separate media type information from media size information, particularly for the sizes that can be both envelope or plain sheets (ISO B and C series at least).
> On Feb 17, 2022, at 10:11 PM, ISTO-PWG Internet Printing Protocol workgroup discussion forum via ipp <ipp at pwg.org> wrote:
>> Hi there,
>> Sorry if I'm asking a question that is obvious to some, but today I was asked to explain why the MSN2 names for envelope media sizes don't have "envelope" as a substring, and I was unable to do so. Why is this? The "size-name" portion would seem to allow, for instance, "iso_c5_162x229mm" to instead be "iso_c5-envelope_162x229mm".
>> Thanks for any help,
>> Smith
>> /**
> Smith Kennedy
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Michael Sweet