I did get that eventually, but I missed the side-note in my first read through, and it was lumping two concepts together under the name of one of them that I fo

I did get that eventually, but I missed the side-note in my first read through, and it was lumping two concepts together under the name of one of them that I found confusing. If, in your description of the three buckets, you called the middle one something else like "Platform-dependent behaviour" or whatever, I think that would have been less weird. You'd still only need one short paragraph to say it's compiler/OS/hardware dependent, covers implementation-defined and unspecified, and is not the focus of the article.