N403 in X12 270/271: A Field Guide to Postal Code
N403 can hold either a five-digit ZIP or a nine-digit ZIP+4, and a lot of test data only ever generates the five-digit version — which means the extended format never gets exercised until a real payer file shows up with one.
N403 is the third element of the N4 segment in an X12 271 response — a coded field that carries the postal code for the address described by the surrounding N3 and N4 segments, in either five-digit or nine-digit ZIP+4 format.
What this element carries
Example
N403 = 43215, a five-digit postal code following the city and state. Synthetic 271 response data.
Where this trips people up
A field defined or displayed with a hard five-character limit will silently truncate a nine-digit ZIP+4 value down to just the base ZIP, dropping the delivery-route extension without raising any error. The truncated address still looks perfectly valid, so the loss only becomes visible later, when a mailing or verification step that depended on the full nine digits gets a less precise match than it expected.