Race plastic is fiberglass, lighter and meant to withstand crashes. It doesn't have holes for headlights, tail lights,or turn signals.
OEM are for the street riding. They have mounting holes for all lights and signals, they're thicker, usually heavier because they're thicker. They are some kind of plastic which are brittle and don't crash as well but they are made in several small pieces instead of 3 large pieces like track plastics.
CCS & MCRA #81