Just a trade secret, just about everyone uses a site called realoem.com
http://www.realoem.com/bmw/showparts...67&hg=51&fg=50 Navigate through to pick your model or use part of your VIN to check the parts.
Depending on your interior you can pick the panel accordingly with the part number. A quick search of the color code abbreviations will tell you what they are. As well as cross referencing the part number here
http://www.realoem.com/bmw/partxref.do will tell you that the 318ti door panels are unique