They also refer to the special releases in Challenge that would outline a new game line. For instance, when Space 1889, TNE, or Dark Conspiracy was launched. Challenge did a multipage spread dedicated to the background on the milieu.
The special supplements have become a bit of an old fashioned idea, now in the age of the web. But, I still think it is useful for publishers to thrown in "freebies" now and again, as it builds product recognition. I know that Traveller20 lite was a big success.
The problem is that most FGLS keep these freebies behind the desk and only offer them to paying customers. So it does not bring in newbies. So I fear we are back to square one...relying upon the web.
...but I would want see more illustrative material for Traveller and the current gallery needs more art. As for me, it is the art that sells the game. Won't say more as I know most know my views and it can be debated endlessly.