The USPS Intelligent Mail barcode - IMb, once called OneCode - is the barcode of the United States Postal Service. It carries the ZIP code and a tracking code for each mail piece, and since 2013 it has replaced PostNet and Planet for the automation prices.

Tracking code and routing code

Tracking code only

What it holds
Characters	Digits only, with no spaces or hyphens.
Length	The 20-digit tracking code, then the routing code - the ZIP code - of 0, 5, 9 or 11 digits: 20, 25, 29 or 31 digits in all.
Check digit	A check code built into the bars by Labeljoy.
Readable text	The digits in their groups: barcode identifier, service type, Mailer ID, serial number and ZIP code.

The tracking code
Barcode identifier	2 digits; the second is 0 to 4.
Service type	3 digits.
Mailer ID	6 digits, or 9 if it starts with 9 - USPS assigns it.
Serial number	The remaining 9 digits, or 6 with a 9-digit Mailer ID.

The rest of the settings - text position, quiet zone, color - are in the Barcode section.

What goes wrong
Invalid tracking code length.	Fewer than 20 digits.
The routing code must be 0, 5, 9 or 11 digits.	The digits after the first 20 aren't a ZIP code of a valid length.
The second digit of the barcode identifier must be 0 to 4.	As it says.
Input data too long.	More than 31 digits.
Numeric only data expected.	The value has something other than digits.