X.400 is a message transfer protocol. In principle X.400 is very similar to e-mail, but utilises a dedicated network rather than the internet. As with email, messaging via X.400 takes place via mailboxes. Each X.400 mailbox is identified by a unique address that is structured in a similar way to an SMTP e-mail address, but more complex and often longer. While messaging via X.400 is costly, the fact that X.400 uses a network that is physically separated from the internet and in which every participant is authenticated provides a good level of security.

Extensible Markup Language (XML) is a digital file format that is used for encoding data and exchanging it between computer systems. XML documents consist of tagged elements and attributes that define the data's structure and meaning. Significantly, in XML documents these elements are both human-readable and machine-readable.

XRechnung is an electronic invoicing standard used in Germany for exchanging invoices between businesses and public authorities. It is based on the European standard EN 16931 and facilitates the structured exchange of invoice data in a machine-readable format.