The module is called EDI. - "Provides a common EDI platform that other Applications can use. OpenERP specifies a generic EDI format for exchanging business documents between different systems, and provides generic mechanisms to import and export them."
It was working differently at 6.1 - some EDI features were replaced by the improved 7.0 portal.
For background, check out https://doc.openerp.com/6.1/it/book/3/3_6/invoice_payment/
I haven't checked out how it works at version 7.0, but from the release notes:
The EDI mechanism is mobilized when a user shares an OpenERP object (e.g. a sales quotation) with a customer. The mail message is the vehicle used to transmit the EDI file; a file that is signed with an auto generated key pair. The mail contains the link towards the object, allowing the customer to directly view that object in the originating OpenERP system.
Furthermore, the mail contains the JSON attachment, which permits the importation of the data into the target system. OpenERP 7.0 automatically adds the instructions explaining how to import the document.
Typically, the customer can click on the link to visualise the data in a form view. An accompanying EDI panel provides options to either import the EDI message using the URL or get the JSON data to perform a manual import.
Alternatively, the customer can proceed by forwarding the message to the generic email address of his OpenERP instance. When the latter receives the EDI message and accepts the authentication information of the originating instance, the file will be imported. When authentication fails, the EDI message is held in a queue, ready for the authorized administrator to either reject the message, accept it once or to approve it. In the last scenario, the key and the originating OpenERP instance identity will be stored in the receiving database, thus enabling automatic acceptance of future EDI message from that source.
According to OpenERP, "It is planned in 7.1 to provide a new EDI export system via email notification and attaching JSON EDI blobs to outgoing emails."