NPA is an abbreviation for "Numbering Plan Area", what folks in the NANP (North American Numbering Plan, Country Code 1) generally refer to as an Area Code. Independent of this is the nomenclature used in Asterisk and elsewhere for parsing a dialed (or received) number. If you have a look at the Asterisk DIAL command reference page, you will see that N represents a digit from 2 through 9 and X represents any digit from 0 through 9.
The SIP URI in your example is a mash-up of these two independent conventions, and its length is three digits too long as well. 1NXXNXXXXXX is generally used to represent an NANP phone number.