The Intelligent Mail Barcode (IMb) is the USPS barcode standard for letters and flats. It replaced the older POSTNET barcode in 2013. The IMb encodes a Mailer ID, a Service Type ID (which signals what kind of mail it is — First-Class, Marketing Mail, Periodical, etc.), and a unique Serial Number — enough to uniquely identify a single piece of mail in the USPS network.
When mail moves through USPS sorting facilities, every scan against an IMb generates a tracking event. The sender can pull these events down via USPS Informed Visibility, getting in-network tracking similar to what UPS or FedEx provides for parcels — entry scans, processing scans, out-for-delivery scans.
IMb is required to unlock the deeper USPS work-share discounts. A piece of mail that arrives at USPS without an IMb is treated as retail mail and priced accordingly. A piece with a full-service IMb (correctly encoded, unique Serial Number, paired with electronic documentation) qualifies for the full work-share discount.
For shippers running statement mail, fundraising appeals, or any time-sensitive direct mail, IMb tracking lets you know exactly when mail is landing in mailboxes — which means you can sequence your call-center staffing, your email follow-up, or your hot-lead handoffs around the actual delivery curve rather than guessing from "it shipped on day 0."