The rails in Memphis would turn you around at gate, if on a 20ft container/isotank Placards had to be as high as possible so that when they set them in the cradle the placards wouldn’t become blocked from view. Used to get to stack those tanks at Boasso.
I noticed that rail containers always had the placards close to the top. We hauled for a private ocean going company that runs a weekly schedule from the US to Europe and back. The placement of the placards didn't matter to them. They don't use trains here or in Europe.
I've always been told to use the red ones for diesel. And we use the white ones for 4-oil and atmospheric gas oil. I'd have to check the book for specifics, but I think the flashpoint of the material is what makes the difference.
Flammable liquids have a flashpoint of 140 degrees or less. Combustible is 141 degrees and higher. Flashpoint is the temperature that the liquid begins to release vapors that when mixed with the ambient air can be ignited with a spark. Federal Register :: Request Access
The "Marine Pollutant" or "Dead Fish" is actually a label not a placard. It's only used when crossing a body of water. For the most part's not used in the US, but there are times it's required.