NFPA 54 does not reference that in the body of the code. They do say a dv less than 10k btu is 6"clearance to a openable door or window. A DV less than 50kbtu is 9" too door/window and a DV in excess of 50kbtu is 12". The number you want to make sure of in MA is 12" clearance to ground or LOCAL snow load and the AHJ can make you dance. Call the town inspection services as that clearance varies by town in MA. Beyond that on a DV appliance, the code relies on "manufacturers instructions" and inside corners should be listed. You want as much as you can get on an inside corner to prevent recirculating flue gases.

I wonder why most clearances are documented to within an inch and this one is not. Does it have to do with regional weather patterns? I assume the clearances that are documented are based on actual testing.