HUD's interest would be that park models are in fact being used as permanent homes. There are colonies of them in Florida, for one place. Of course, other types of RVs are also used as permanent homes. Thus it becomes a "so what?" question.
It goes back to when there were no standards for manufactured housing, and people bought house trailers, later called mobile homes, to get around the cost of houses built to construction and safety codes, which most places in this country include standards about minimum size and amount of space per occupant.