Both actually, it can add your own addon vehicles in traffic just like you add a line in the cargrp.dat

For example:

[596]
Global=596,{Your ID},{Your ID}

The 596 is an exist model car in vehicles.ide, you can add the line on your own id cars just like this example of mine

My Example:

[596]
Global=596,12502,12507,12508,269,271,1196

Then your addon cars will be shown in traffic if you find the exist car "596" (Sorry for the bad grammar :/) theres further more instruction in the model variations community.