I'm guessing a couple things...
MAYBE... (not certainly) it's an ACME thread that they don't care what class it is? (Lke N2 or N3 or N4 etc...)
MAYBE... it's not NG but NF?
MAYBE... this note was some sort of tolerance and they're expressing a "No-Go" situation?
Would need to see the print or have more info to have a clue.