This article is from the Pratchett Newsgroups FAQ, by Orin Thomas firstname.lastname@example.org with numerous contributions by others.
An annotation (usually marked [A]) of a Terry Pratchett book is a
reflection of a small "inside joke" used in the text. An example of
which is the "Soul Food Cafe" scene in "Soul Music".
Something written or filmed after the publication of a TP book obviously
does not constitute an annotation. The television show "Due South"
might have been influenced in some way by "Guards! Guards!" but it is
impossible for it to have happened the other way around.
You might also want to consider how you phrase a prospective annotation.
Some annotations sound more like allegations of plagiarism rather
than a helpful attempt to point out an inside joke. Such allegations might
even be considered libelous.
When posting an annotation it would be helpful if you included:
1) The original text you are annotating (TP's) (include page number)
2) The year of the source of the annotation. (Blues Brothers (1981))
3) As close an approximation as you can get of the text of the
source. (i.e. writing out the conversation in the Soul Food Cafe)
If you are worried that your annotation may have been covered before I
suggest that you trawl Dejanews with the original "TP" text. As we
would like you to quote the original when annotating anyway this means
that simply by searching on the original text you should be able to
find any extra commentary.