Your confusion @cip
comes from the moment you are using terms that in a regular conversation means one thing, but in the law means a whole different thing.
General terms of copyright laws state that any material under copyright protection can't be used in it's original form for any different purpose than the initial (that includes how it has been distributed) neither manipulated (ANY kind of manipulation) to create another content without the permission of an author.
The few only exceptions to those general terms exist with the sole purpose of protect human rights far more important and prevalent than the legal copyright. For example, it's allowed to use copyrighted material without any author's permission AND in a very limited way
, in order to talk about the existence of that original material. In the case of a video-game, to use screenshots or examples
of videos or soundtracks of the game in order to do a review, a tutorial, an educational content for a classroom, or an opinion article. But no human right or any other legal right protects us from taking content of other authors to make a mod for a video-game. So, in the case we are talking here, we have to follow the general terms I stated in the second paragraph.
Now, if you exactly and precisely ask: "Can be legally EDITED the content of a DLC to ADD it to a mod"
, the answer is NO. It can't be EDITED (manipulation of the content) and it can't be ADDED to the contents of a mod (edited or not), because that would be unauthorized distribution of copyrighted material. End of (legal) discussion.
If you exactly and precisely ask: "Can a modder create content of it own that will work combined with original copyrighted content that will already be installed in the same system than my mod."
Then the answer is YES; you are neither manipulating the original content neither distributing it with your mod.
So, if when you talk about "edit" or when you talk about %, you are using non precise terms that lead to confusion thus to contradictory answers. Edit is manipulate, edit is not take blender tools, look the original files and then create a file that defines where the HONK! sound of a train would be located (for example). And the % of what amount of edition will be in a mod is irrelevant, NONE is allowed. That's why you, when talking about %, no matter how hypothetic they could be, confused people. There's only two percentages that matters: 0% manipulation and distribution, 100% original content.
When you link your mod with any SCS content, you are not manipulating neither editing SCS content. So, it' doesn't mater if your mod has a 1% or a 99% of LINKED relations to SCS copyrighted material (and the other 1% is a commented line with your name on the mod files). That would still be a 100% original mod of yours with 0% others authors content. So, it will be perfectly legal and fine (and SCS give us the tools and info necessary to ease our work in this way).
Hypothetical over-simplified example: Let's suppose I want to start to do mods making dirty skins for trailers. But I'm so lazy that the only thing I do is to copy SCS texture files, add it some "dirt" with the paint, put the changed files on a mod structure and publish it so all the players could praise me as the new God of mods.
WRONG. I would have infracted the copyright law in two essential terms: manipulating original copyrighted content and distributing it. Now, let's suppose I learned to use some more proper program than the paint, and I create some kind of transparent layer with the dirt that my mod will apply over the SCS skin. I put both thing on my mod version 2.0 and publish it. WRONG
I would still have infracted the copyright law because I would still be distributing copyrighted material without permission. Time later, let's suppose I finally learned to "code" the SCS files, and now I did a mod v3.0 that links the original skins to whose I relate my layer with the dirt. The mod I distribute doesn't have anymore the files of SCS on it (even if the reason is just I'm so lazy to copy them, what matters is the SCS files are not on my mod
). RIGHT. That would be the way to do a 100% legal mod. Even if , instead for ETS/ATS, you were modding for a game published by the most infamous game company, one that would handle relations with their players on the worst way possible; they couldn't legally do nothing to avoid you do a mod in such a way.
Any other questions, of derivatives of that I explained you (if your brain arrived alive to the end of the wall of text
); all those infinite discussions you could find all over Internet, have nothing to do with the state of the law. They are about moral terms, brainless rants, list of wishes of how some people would like to change those laws and so on. But that's not the topic we are talking about here.