Car customisation refers to a scrapped feature in Destruction Derby Arenas that was going to allow the player to modify their vehicle by fitting different unlockable upgrades to different sections of each car. It was revealed in pre-release versions but removed in the final retail release.
A glimpse of what it would have been like is shown in the game's trailer, and is even still advertised on the game's box (see image), including the American release despite occurring at least two months after the European one. It was cancelled/removed for reasons unknown.
There is speculation that by digging through the games files, you can locate this feature, and activate it. This has not yet been confirmed.