This post is a Wiki that can be edited by any regular user.
The following links mostly apply in the US.
Many works and resources nowadays may select a Creative Commons license (CC) and depending on which license is selected, the creator may inherently grant rights to use a work (writing, music, art) and specify whether it can be used commercially, whether it can be transformed or modified, and how the original creator must be credited in the new work.
Works with a CC license can also have different rights available outside of Creative Commons stipulations. For example a piece of music listed CC-BY allows free use so long as the creator is credited in the new work, however the creator may negotiate separately that an entity can pay for the rights to use the work and not credit the creator.
It’s important to note that most of these articles are US-centric, or entirely about US law only. Fair Use in particular is a US-only concept. Other countries may have different-named concepts that are similar, or may not have an equivalent.
I know Hanon put this together to help with questions re: your ability to enter fanfiction or derivative works in IFComp, and for that purpose, I expect it does the trick. Because it extends what IFComp’s own rules say about it, and IFComp is US-based.
I’m just saying that if you’re considering the legal situation for your derivative work when it’s outside of IFComp, the US-specific parts of the information won’t hold if the work you’re deriving from, or you (or furthermore, your work’s carriage service or carriage service’s hosting country… hello giant complexity sinkhole!) aren’t from the US.