Wikipedia:Sources must be out-of-universe

What is a Sourcebook?
In this context a "sourcebook" doesn't mean a book used as a source. Instead what it means is what most people think of when they think of a book for a table-top role-playing game. It's a book that contains rules as well as lists of items, magic spells, monsters the players might encounter, devices they might find and so on.

These types of books contain information on a lot of fictional devices, concepts and other minutiae of the fictional setting the game takes place in.

It should be noted that books like the Star Wars Technical Manuals are a sourcebook just as much as books like Dungeons and Dragons' Monstrous Compendium or Toon's Acme Catalog. It need not be a pen-and-paper RPG that's the subject: the important distinction is that the work is mostly a list of fictional items from an in-universe perspective

Why are "sourcebooks" a problem?
Because these types of books exist, it may appear at first that an article on a very minor item has a staggering number of citations and references. Sourcebooks rapidly can create an echo chamber of sorts, re-reporting and re-releasing the same information over and over without actually saying anything significant at all.

Take as an example a common pistol from Shadowrun: the Ares Predator. It appeared first in the first edition core rulebook. It was also mentioned in that edition's gun compilation, the Street Samurai Catalog. As editions changed it was in the second edition of both of those books. But come third edition the Street Samurai Catalog was replaced by the Cannon Companion, that's book #3 that can be cited. Then in fourth edition the title of the gun sourcebook changed once again. That's a total of 6 books none of which say a single thing about it other than what it is and some in-universe information about its popularity with mercenaries in the setting and its reputation for reliability.

The solution
Because of this effect it is vitally important to make sure that the sources say something other than purely in-universe information. A short description of what an item does is important context for an encyclopedic article, but (as wp:WAF says) the primary frame of reference at all times should be the real world. WP:NOT also states that information presented in an encyclopedic article should not be primarily of use to someone playing the game (and by extension reading the series, etc.).

an example
The Corellian Engineering YT-1300 Light Freighter from Star Wars is unimportant as a group of ships, but the Millennium Falcon probably IS notable, even though it's a YT-1300. Any article about the Millennium Falcon should focus on the many toy versions, how it has become an icon of the Star Wars franchise and how it has inspired the designs of space ships from other science fiction series and games and other ships in the star wars universe. Its hyperspace factor speed, what fictional company made its engines, armament and computer systems, variants that aren't shown in any notable book or comic, and anything else primarily important only to people within the Star Wars universe would be Undue Weight given to in-universe sources.