[Pasha Paterson]

A lot of the content we use in AC on SRS comes from the modding community. However, some of the mods that are available are not usable here, because they are not "original work", or are directly ripped from other games.
But what does that mean? Well, it's possible to extract the 3D models for cars and tracks from other games (GranTurismo, Forza, ACC, GRID, etc) and adapt them for use in AC. When this is done for cars, the physics are usually either (1) terrible or (2) copied from existing cars. For tracks, the differences are less noticeable. While these cars and tracks might be pretty to look at, and fun to drive, SRS cannot endorse their use, because this kind of reuse of content from other games is a breach of the copyrights on those games. SRS could theoretically get into legal trouble for endorsing those mods.
Usually, RaceDepartment spends some effort on screening for ripped content, and if you see a new mod go up on RD and then disappear a few days later, chances are good that someone discovered that it was ripped content. Sometimes, content of "dubious origin" can stay up on RD for a while, but generally it can be trusted.
There are some obvious warning signs for ripped content.

• Sometimes the mod creator will openly admit that the content is ripped, and even mention where it came from. Obviously these cannot be used. This situation includes sites or groups who don't say anything about individual mods but have a history of using ripped assets. This latter category includes groups like Guerilla Mods and The Rollovers.
  
• If the mod author says that it was converted from another game that supports mods (like rFactor 2), this might or might not have been done with the permission of the original mod author. You would also have to know whether that original game's mod was part of its paid content, or developed under an official paid deal with the game's publisher. An interesting and very complicated example of this is Shaun Clarke's BTCC car packs; while Shaun was part of the development team for these on rF2, they were developed as part of a commission by Studio 397, and the argument over whether he had permission to "liberate" them for use in AC (his word) was never really settled. They're fun cars to play with in AC, but unfortunately can't be used here.
  
• For car mods, Content Manager includes an "Analyze" feature that will examine the car's data for several key metrics, such as polygon count, skin size, sound pack, and physics data. The primary use of this is to assess possible problems with the mod's performance in the game — for example, if the polygon count is too high, or the skin size is too large, it will perform very poorly in the game session, especially in a race with many cars. The Analyze tool can also be used to spot instances of ripped content, especially when the mod is poorly-made. Absence of LODs, very high polygon counts or skin sizes, sound packs copied from other cars, physics data that is identical to other cars, or any major problems with the car's data definitions, could all be clues that the mod is not entirely original work.
  

There are also some clues that allow you to know when a mod is original content and is probably usable.

• The mod author includes details of the creation process, like screenshots of the 3D modeling software, modeling data, conversations with real-life teams, etc. — basically anything that gives insight into how the mod was created. Creators like aphidgod, Gary Paterson, LilSki and JohnR, Pessio, Phoenix Mods, the MAD Formula Team, and Trained Monkey Modding have done this with all of their original mods. This can give some confidence that the work is indeed original. (TMM even has separate sections of their website for content which is completely original vs. content that uses assets from other places, so that you can limit your choices to their 100% original creations.)
  
• The mod is officially endorsed or made by a real-world team. Specific examples of this include TMM's Cupra Leon Competicion 2020, and the SMP Racing BR03 prototype. (Unfortunately the latter can't be used here because it requires CSP, but it's a very fun car, and you should download it anyway. )
  

For some more in-depth discussion of this problem, see my video about "mod legitimacy" and the infamous Brabham BT62, and my review of TMM's Cupra, where I interview Fuzo about this question and how it relates to the mod.

I hope this post helps to clarify the question of mod legitimacy, ripped content, and why SRS takes such a strong stance on the subject.