Freight classes are based on four things: density, freight stowability, ease of handling, and liability.
A truckload of Styrofoam coolers is the "typical" Class 500 load: a 53-foot truckload of them only weighs 650 pounds (I have hauled these), they're hard to handle because they're fragile, they're relatively expensive for their weight, and their breakability means you're probably going to have a damage claim. So the shipper gets charged an arm and a leg.
At the other end of the scale is a 40,000-pound iron ingot. They're very easy to handle, impossible to damage even if you rolled the truck, easy to drive with and very inexpensive per pound. These would be freight class 50.
Agriculture
Assembled furniture
NMFC Freight Class 70 would be something like acorns or other shelled nuts.
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Usually reserved for used electronics.
Boat, Inflatable, Knocked-Down in a box
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Freight class 200 is "instrument" -> "Scientific"
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