Legally there's a major differance between items like cars which are covered by design patents and trademarks. Artistic works, are determined by protected elements and deriviative works.
When you make a car for example it's legal for a person to come in and make an aftermarket part, provide there is not a unique system of mounting said part. Certain cars might for instance have a special type mounting bracket of bolt head design they use that they own a patented design to. In that case you have to obtain rights to make a part that can interfacce with that design.
Lexus and Saturn both make parts like that, lexus has special engine covers and custom oil filters that don't fit on any other car, so their oil filters can only be bought from their dealers or a offical supplier. They own a patent on the design and there are no generic alternatives. (which is why we couldn't service them at the shop I used to work at)
Legally however there's little they can do if you open up a company that makes a bolt on light for example that you drill a hole in the bumper and srew in it place as there's no propriatary design invovled.
A grey area would be making a adapter device that attaches to the engine and allows you to use a standard generic oil filter on a lexus. Since it needs a special threading and plug in it's design that might mean it's infringing on a protected design.
The laws patent regarding mechicanical and chemical designs are pretty straight forward as there are technical diagrams, measurements, formulas etc that all determine if something is compatible or the same.
The law set for Itellectual Properties and Artistic works are a completely differant ball game though, very complicated and not particuarly clear. The reason for this is that they are often not physical object but ideas and specific expressions of those ideas. You can't claim ownership of the general idea of a robot with a gun, but you can own a particular design of that robot with a gun.
What makes an item unique or how it expresses the idea is entirely subjective, which is why IP stuff is so hard to define in a hard legal code like patents. A painting for instance can be viewed by a hundred people and every person can have a differant opinion of what it means of the artist was trying to say.
Computer code and books are another major IP area, you can't own the numbers that make up code, and you can't own words or letters the only thing you can try and claim is the specific order and combinations of those words/numbers. If it's contested in court the jury has to figure out if those patterns and phrasings are unique or if it's something commonplace that anyone could do, etc.
Comparing aftermarket cars parts in a super simplistic way makes logical sense, but is worlds apart legally.
The problem is that IP issues are covered by subjective law and very tough to have a solid yes or no on.
One of the famous art examples is a french artist who made a copy of the Mona lisa and drew a goatee on her with pencil, grade school style. (added an abreviation in french for: she has a hot ass) He then printed that image on tons of stuff citing that he owned the rights to that piece of "art" because his modifications made it an entirely new work.
The Mona Lisa is still clearly a visable and is the central element in the piece so to some people it's merely a work derived from the original source, but it can also be argued that the piece has been substancially transformed and given a completely new meaning.
You can ask that question to a hundred differant people and you'd be surprised at the variety of answers you'll get, even the people that have a hard yes or no answer from their gut can't really define why without a complex discussion and giving lots of thought as to why it is or isn't a unique work.
The Chapterhouse lawsuit won't provide a definitive answer like people are expecting, no matter how it resovles it's only going to be an answer for that question of the moment and that particular case. Somebody could be doing something in the exacct same manner as CH and get an entirely differant result because the jury sees the works differantly.
In a realistic view it'll help people to better figure out what extent they can push things to without GW raising hell, but unfortunately legally it doens't settle much dispute as to where the hard black and white line is drawn as IP iconcerns are almost all grey. (and it comes down to which side spins it best)