I asked Jerry at Atlas about these as I wanted to buy a few that could fit into my layout as I had no clue to what each would carry. He was nice to reply to me and heres what he posted:
Cars often carry different products over the lifetime of the car
Free running or open lease cars are more likely to carry different products
CRGX and ADMX carry vegetable oils
PLMX is a free-runner and the model is placarded for <1268>
GATX 54xxx is a free runner, the model is on lease to Rexene Corporation and placarded for <2055> styrene
GATX 183xx is a free runner, this model is placarded for acrylate.
IBPX is used for tallow
David Casdorph
Atlas Model Railroad Co. Inc.