Basically, it means that the car has more than one livery (usually, 4), but you have to edit the vehicles.ide file with notepad (or Notepad++) and add the argument (without the quotation marks) "+livery" at the ned of the desired vehicle's line, eg: POLICE, POLICE2, NOOSE and so on. Now it means that when you will select a police car in the car selection step, you will see the other liveries available. Now, if you want to have a livery for a specific region of Liberty (such as Algonquin, Alderney, Broker, etc), you will need to get the ALM mod available on this site.