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
Note: To find and download the document, copy the document number (like NENA-STA-010) from below and search the NENA Standards and Documents page.

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