Actually, offsite links and references ARE allowed, just not to "competing" sites, such as those which sell 3d models or textures, or sites that provide 3D models/scripts, textures, Photoshop/Gimp brushes, etc. for free. Nor are you allowed to post a link to file locker sites, file sharing sites, or non-trusted sites (basically, anything that is not a notoriously known and well-trusted site, such as one's personal domain name)
You are allowed to post links to well-known "trusted" sites such as Youtube and Amazon.

To be fair, as much as I loathe these rules and hold them, and the so called "logic" behind them in the uttermost contempt, I can see the logic and fairness behind banning links to file lockers and file sharing sites (and ONLY them), as these appear to be among the most commonly hijacked sites by malware attackers and warez squatters. A couple of years ago, the most commonly used file sharing site used on Rendo at the time was hijacked by The Pirate Bay (to be specific, the URL had become the page of Pirate Bay's blog)