Creative Commons Licenses.
I found this article quite detailed and interesting. I learned, for example, that attribution used to be just an option out of many for CC licenses, and that with 2.0 it became automatically part of all of them because such a high percentage of people always chose it (the Wikipedia article links to this page explaining the change for the 2.0 license suite).
I also found the table under "types of licenses," "seven regularly used licenses" quite interesting because it connects the licenses with other 'open' and 'free' frameworks, such as the open definition and free cultural works.
The list of court cases towards the end is useful as well., and has the latest case (Great Minds v Fed Ex (2018)).
Altogether the Wikipedia page is relevant to many parts of unit 3, including license types and enforceability. I wonder, for the latter, if a link also to this list of case law on the CC wiki would be useful (though it was last edited in 2017).