Did you ever get this answered? The copyright.gov site covers it pretty well. I dealt with this a lot over the years for employers and online content and their lawyers always promoted permission, citation and full crediting for any use of material that was used, when it was being quoted verbatim and specifically if it was in the format of the creator on even the most benign and innocuous material and circumstance. But it also depends a little on 1. your moral and ethical values and 2. how deep those ethical pockets are. In a worst case scenario the worst that can happen to most of us is a cease-and-desist order, which any licensed attorney - well, anyone really, can issue. But who wants that? So your question is valid.
Two scenarios I dealt with were typical - if it was clearly for commercial use permission was required and cited in a set of footnotes at the end of the content, document, paper, etc.
If it was use of a set of words and phrases that came from a piece of reference material like a dictionary or industry standard reference source - it was assumed that non licensed use was legal unless otherwise stated in the source material itself. It was still always cited and credited as source material in the footnotes.
So if someone wrote .... "the definition of the term 'duty of care' may apply here, which in common legal definition is 'a requirement that a person act toward others and the public with the watchfulness, attention, caution and prudence that a reasonable person in the circumstances would use.' "....and it would be cited in the footnotes for the source of the quote.
This can be expressed in language many ways, I could say the same thing but write "as it states in the online source dictionary.com.law" blah blah blah.......which covers the citation (and would still get included as a reference source in footnotes listing) but it would assume that non licensed use is permitted......
And your question was generally around "but is it?"
You're on the right track. I knew the answers to that for most of the situations I encountered and understood the application of them, so I could actually do a first review of the content if it hadn't "been through legal" yet, and then forward to them for review and in nearly every case they hit that nail again and again with the answer "Let's make sure we have permission and include the statement in the footnotes".
I'd consider contacting some reliable sources or an actual attorney to go over your questions. If it were me, and I was using publicly available reference material for the purpose of research, teaching and academic advancement I'd cite and note a nice clear thank you to the sources in my foreword and/or in the footnotes. If I were collecting a charge or donation for the material I'd written for the usual "covers cost and handling", I'd handle it that way too, thus declaring openly my intentions.
Run it by an attorney for fast relief and get their answer in writing.
Just the facts, please:
https://www.copyright.gov/fair-use/more-info.html
Some attorney talk about the facts:
https://www.janefriedman.com/the-fair-use-doctrine/