Thank you for that. I didn't realize that NIST used the term officially.
I find that in 5.3.2 of NIST Special Publication 811 (Guide): The following statement occurs: "There are many units besides CGS units that are outside the SI and not accepted for use with it, including, of course, all of the U.S. customary (that is, inch-pound) units."
Note that the word "customary" is not capitalized, so it appears to be a descriptive term, i.e., the units that are customarily used in the U.S., rather than strictly the proper name of a system (U.S. Customary), although the term is commonly used in this way, as indicated by its common abbreviation, "USC."
Martin Morrison
