AS2 (Applicability Statement 2) is a specification describing how to transport structured business-to-business (B2B) data securely and reliably over the Internet. Security is achieved by using digital certificates and encryption.

AS2 uses the HTTP(S) protocol to transmit data and the S/MIME standard to encrypt and sign the payload. The payload is sometimes referred to as the "message" or "document".

AS2 is a peer-to-peer (P2P) protocol where both sides of the connection need to agree to the connection and provide each other with the required information for successful data transmission. These connections are sometimes referred to as "AS2 partnerships".

AS2 was designed for business-to-business transactions, such as Purchase Orders, Invoices, Shipment Statuses, and other business documents, so the two businesses at either end of the AS2 connection are referred to as trading partners.

About Message Disposition Notification (MDN)

A Message Disposition Notification (MDN) is a digital receipt, returned by the receiver of the AS2 payload (message), to provide confirmation of delivery to the sender.

The Message Disposition Notification (MDN), when digitally signed, also provides non-repudiation.

The MDN can be used to return a successful delivery notification or a failure outcome. A failed MDN will usually contain details about why the delivery failed.

Despite being an optional component of AS2, the use of MDNs is widely adopted, and often mandated, due to their use in resolving disputes between trading partners.