The term "fair use" is written into copyright law (title 17). Judges will reference the text of the law when deciding cases.
Let's go back to Dan's point about classroom use...
I'm teaching a class to our gunmakers guild. There are 30 paid members. Non-members are not allowed. I'm providing a free handout to each member. The handout has copies I've obtained from various sources. I will quote the sources in a bibliography contained in the handout. Under fair use can I make and distribute 30 copies of the handout without getting copyright permission.
The use is educational in nature... which leans towards fair use.
It's not a "course packet" that students have to buy. So I derive no income from other's copyrighted material. That also leans towards fair use.
The distribution is limited. Which leans towards fair use.
The nature of the material is historical, and therefore "factual" in nature. This leans towards fair use. If it was "imaginative" in nature (songs, movies, etc.), it would lean against fair use. For example I couldn't show "the Gunsmith of Williamsburg" video in class because that would be a "public display" of a copyrighted movie. I would need permission from the copyright holder.
I'm limiting it to a couple of photos or quotes from each source. Therefore the quantity is limited, which leans towards fair use.
Is there a permissions market where the copyright holder could charge me? Maybe... maybe not. But this one factor doesn't trump all others. I feel safe using the material under the "fair use" provision written into the law. Why? Because taken as a whole the use leans in favor of "fair use".
For practical purposes, universities often use (non-legal) "guidelines" such as... "one chapter" from each source. Generally they follow court cases to determine what they're "guidelines" will be. They do this to stay safe from legal challenges. The big case 20 years ago was against Georgia State University. I won't quote everything here... you can look that up yourself... but we followed that case in detail as it was being argued.
A really good source for information is at the University of Texas website...
https://guides.lib.utexas.edu/copyrightMy own philosophy... I make a "good faith" effort to stay within the law.
By the way... "good faith" is also a quote from title 17.
I won't post stuff online. You need to buy the book as I have done. However, I do feel comfortable distributing copies of some stuff to individuals if they ask... and if it's of limited quantity (i.e. one chapter or a couple of photos). I do this because it enhances our education of the subject... which... taken as a whole... benefits us all. And... it's my legal right to do so under the "fair use" provision written into copyright law. The rights of the author, photographer, or publisher are not absolute.