This article is from the Pratchett Newsgroups FAQ, by Orin Thomas firstname.lastname@example.org with numerous contributions by others.
Several other "bracketed capitals" have cropped up on alt.fan.pratchett
These are not "official" and sometimes I wonder if people make up new
ones just to keep life interesting. [I], [R], and the occasional [A]
are simple ... anything more is bordering on the anal-retentive. You
shouldn't constantly need to refer to a manual to read Usenet!
Karen posts an FAQ explaining all of this.