They are used to mark class.
In the United States the terms "trailer part trash" and/or "white trash" are a used in the US mainly derogatory to describe class difference. The people living in trailers or trailer parks are often of humble financial status to put it mildly. They are usually used for Caucasian Americans in a derogatory manner. It is the similar to the terms "redneck" and "hillbilly".
