So basically what it should be doing is, it checks if a certain condition, in our case, it checks if a part has been installed then we can install only one type of part, but if it hasn't then we can install only another type of part. For example (From Oskar) : Installing a wider wheel when a wide fender is installed, but a normal wheel when it is not. Just an idea for customisation from Oskar, decided to ticket it for future, if it can't be done now.

What you're asking for here is additionalParts, or customParts that have specific subParts. Both can currently be used to do what you want.