Au contraire.

First of all the size of the car. Two or three bay covered hoppers carry high density products such as cement, kaolin or lime (or in the good old days, carbon black). Four and five bay covered hoppers carry lighter materials such as plastic pellets, grain or flour. The cars carrying powdery or plastic products usually have circular loading hatches and either pneumatic or gravity outlet gates (more often, pneumatic). Grain cars have long loading hatches and have gravity outlets. Rule of thumb if you are modeling - substance that are powdery or blown into the car = circular hatches, grain(which doesn't flow that well) = long trouths. By the same token - substances that are unloaded pneumatically (again, powdery or light weight materials) = obviously, pneumatic outlets. Things that flow into underground hoppers when unloading(wheat, corn) = sliding outlet gates.

Now, what is in what car? If you buy a decorated car, usually RR owned cars (e.g., UPRR)or cars labeled for some agricultural owner (e.g., Wagner Mills, ADM, Cargill) are grain (some exceptions), if labeled for something like Dow, or duPont, probably plastic (although duPont also made titanium dioxide for paint & paper making - which is a high density powder material). I think most GATX, UTLX and other leased cars are used for plastic pellets, but some may be used for grain. If it is labeled Lone Star or CEMEX it is for cement.

-- John