From NENA Knowledge Base

NXX is most commonly used to refer to three-digit code that forms the second part of a 10-digit North American phone number (digits 4-6). This is also known as the "central office code" or "exchange". Traditionally, in the age of competing phone companies, each NXX is assigned to one company.

Relevant NENA Documents

NENA 03-008, NENA Standard for E9-1-1 Default Assignment and Call Routing Functions