The NXX is part of the seven-digit phone number that follows the area code (NPA) in the North American Numbering Plan (NANP) and helps the phone system route calls efficiently by identifying specific switches within a geographic region (NPA). Here’s what it means:

• N: The first digit of the NXX, which can be any number from 2 to 9. It can’t be 0 or 1 because those digits have special functions (e.g., 0 for operator assistance, 1 for long-distance dialing).
• X: The second and third digits of the NXX, which can be any number from 0 to 9.

So, NXX essentially represents the central office code or exchange code. It helps identify the specific local exchange or telephone switch that the number belongs to within a particular area code.

For example, in the phone number 555-1234 (where 555 is the NXX):

• “555” would tell the system which local switch (or exchange) to direct the call to.
• The next four digits (1234) identify the individual subscriber or telephone line within that exchange.