I also think Moose has it right, but I would like to see the regulation in the FMCSA book.
I honestly don't know if it's in the book or not, and I'm too lazy to look it up, but I would imagine that it's not in there. The reason is, the FMSCA regulations themselves already define HAZMAT, and the book deals with HAZMAT as
already defined in the regulations.
The test question is pretty straightforward, as it asks about "hazardous materials". If it's "hazardous materials" then it's HAZMAT (and thus placardable according to the FMCSA definition), and you need the endorsement.
And if it's not "hazardous materials" then it's not, and you don't.
Even though I use the term all the time to delineate material which would otherwise be placardable in certain amounts but not placardable in other amounts, there's really no such thing as non-placardable HAZMAT. It's either HAZMAT or its not, and if it is, then it's placardable and you need the endorsement.
It is just a good idea to have that endorsement. You can never tell when you might have some HAZMAT "slipped" onto your truck without you knowing it.
It happened to us. We were loading at the CDC heading down for Hurricane Gustov. We were told NO HAZMAT on this load. While loading we noticed HAZMAT stickers on some of the freight. It made them mad but we required them produce new paper work reflecting the O2 canisters in the kits. They complained that no other drivers made them do that. Oh well, do it right or don't do it.
Of course, just because some or all of the freight has HAZMAT stickers on the boxes, that doesn't mean the load itself is HAZMAT. It might be in certain quantities (more than 1000 pounds, for example), but not in the quantities you are being loaded with. A 5-gallon bucket of HAZMAT paint is gonna have a HAZMAT sticker on it, but it ain't HAZMAT unless there is more than 1000 pounds of it being loaded onto the truck. A single 5-gallon bucket of HAZMAT paint loaded amongst non-HAZMAT freight, even if the total weight of everything is well over 1000 pounds, doesn't make the load a HAZMAT load.
I once picked up two skids, one was 800 pounds of wet batteries, and the other was 1200 pounds of tools and other hardware. Total weight was 2000 pounds, but it wasn't a HAZMAT load because the "hazmat" portion of the load wasn't a HAZMAT quantity.
But I agree that it's just a good idea to have the endorsement, even if you never haul HAZMAT. It lets you know what you're looking at, the freight, the bills, and the placards on that wild and crazy driving truck in the lane next to you. There are a lot of shipping clerks that don't really understand it, and they'll try to load you with placardable HAZMAT when the bill isn't marked as such, and they'll incorrectly mark something as HAZMAT when it isn't. If you have the endorsement you'll know and be able to spot those kinds of errors. For example, if you are picking up freight that is marked as HAZMAT on the paperwork, but it's not HAZMAT freight, it's illegal to just go ahead and haul it anyway without first having the paperwork corrected, just the same as it's illegal to run with HAZMAT placards if you aren't actually loaded with HAZMAT.