Many people will first be exposed to threading tool features in the form of a quote request, or order, from a customer. Example: Please quote 6 pieces of a 1"-20 NEF HSS 2B LH GH-3 6-FL PL tap.
Dissected, the customer has defined this tap as one with a "major" or "outside diameter" measuring 1 inch, with 20 threads per inch.
NEF, or National Extra Fine, is a thread-form symbol that refers to a National Standard for this combination of diameter and threads per inch (TPI). Others are UNC (National Coarse), UNF