AS4 is an open standard for the secure and payload-agnostic exchange of business-to-business documents using web services. Secure document exchange is governed by aspects of web services security, including XML encryption and XML digital signatures. Payload agnosticism refers to the document type (e.g. purchase order, invoice, etc.) not being tied to any defined SOAP (Simple Object Access Protocol) action or operation. AS4 is increasingly recommended as the successor to AS2.