I believe the USD refers to the Federal Reserve Notes that masquerade as our "dollars" and the UST refers to the US Treasury... which is the only one that has the right (by way of Congress through the Constitution) to print currency. The privately-owned "Fed" does not have that right and has been illegal under the Constitution since its inception in 1913. Hopefully, new asset-backed currency should be coming out via the UST not too far down the road! Notice the recent change in the $100 bill... a bit of easing into the UST with its "half and half" design. So, the UST will refer to both the Treasury and the Treasury dollars.