Are we discussing whether a material is Haz Mat or whether it requires placarding and special licensing?  I'm not sure all three of them are the same!  For instance a material may or may not need placards depending on what the substance is and how much you are carrying.  I forget off the top of my head the quantities, but I think you can carry 1000 pounds of propane AND the container it is in without being placarded.  In other words 5 full 100 pound cylinders no problem, 6 you need placards, haz mat endorsement on your license, probably tank endorsement.

Anydrous Ammonia is always a hazmat.  Propane is always a haz mat.  But any quantity of NH3 needs placards where you can haul some LP without placards or endorsements.

This is why you get so many answers.  We ask simple questions that do not have simple answers.

But whoever gave a flat answer of anything over/under a set number of gallons is wrong.  Period.  Unless the one asking the question specified a specific product.  That's why this is so confusing.  There is a definite non-arguable answer ... except for that one case.

edit:  I just stumbled across a  DOT brochure that give a quick overview of this.  See https://hazmatonline.phmsa.dot.gov/services/publication_documents/MO...   If you have time to ponder go to https://hazmatonline.phmsa.dot.gov/services/publication_documents/MO... and look around.