I agree there is some fuzziness. Long after we wrote this I read:

https://www.w3.org/2001/tag/doc/versioning#iddiv371153984

which has a very succinct explanation of forwards and backwards compatibility as it relates to producers and consumers.

It's high time we did a second draft of SchemaVer which explains it in terms of forwards/backwards compatibility; the actual behavior of it (when to bump etc) would barely change.