Of the four fields that Jim mentions, two can be printed as a part of footnotes/endnotes and two cannot.
In some ways the names of the four fields are meaningful and are suggestive of how you might be expected to use them. With respect to what RM calls the Master Source, there is Master Text which is divided into two memo fields called Source Text and Source Comments. Neither of these two fields can be printed as a part of footnotes/endnotes. Some users would probably be inclined to call the "Master Source" simply the "Source".
With respect to the rest of the sourcing data, there is Detail Text which is divided into two memo fields that you might think would be called the Detail Text and the Detail Comments. However, these two fields are actually called Research Notes and Comments. These are the names of the fields from the Edit Source screen, and the same fields have other names in other places in the user interface, something that badly needs to be cleaned up. Either or both of these two fields can be printed as a part of footnotes/endnotes. Some users would probably be inclined to call the rest of the sourcing data simply the "Citation".
For a book, the Master Source typically contains the title and author of the book, along with the associated publishing information. The Master Source is expected to be reused for many, many citations. The rest of the sourcing data would contain the page number. For other genealogical records - birth certificates, death certificates, marriage certificates, obituaries, census, and the like - which part of the sourcing data is contained in the Master Source and which part is contained in the "Citation" data is often disputed between source splitters and source lumpers.
I actually don't find the names of the four memo fields to be very meaningful or useful. I simply remember that there are four of them, that two of them are associated with the Master Source and two of them are associated with the "Citation", and that only the two that are associated with the "Citation" can be printed out as a part of endnotes/footnotes.
A current weakness in RM's sourcing model is that if, for example, you create a citation and memorize and paste it 29 times, you will then have 30 different instances of the citation. If you then find a typographical error in one of the memo fields associated with that citation, you have to correct the error 30 different times. For any data that's in the Master Source, any error only has to be corrected 1 time and the error automatically becomes corrected in all 30 citations. For that reason, I have become an extreme source splitter, and I enter 100% of my sourcing data into RM's Master Source. I don't presently print any memos along with footnotes/endnotes. But if I did, I would have to write a (very simple) SQLite script that would copy every Source Text field into every associated Research Notes field. If I found an error in a Source Text, I would correct the error in the Source Text fields and rerun my script.
If you enter text into either of the memo fields associated with your "Citations", there is an option to print them or not as a part of printing reports. But if you haven't entered any such text, then enabling the option when printing a report obviously doesn't do anything.