XML (eXtensible Markup Language) is a text-based format for representing information in a self-describing hierarchical structure. An XML document contains entity tags, which are usually paired into an opening and closing tag. Entity tags are contained within a top-level root tag pair. The data between entity tags is usually a mixture of other paired entity tags and associated data, or simply data. These entity tags consist of character data between the '<' and '>' characters. The structure of an XML document can be strictly enforced by the use of a DTD. A document type definition is a description of how XML entity tags may be arranged in an XML document based on the definition. XML parsers can use the DTD to test whether an XML document conforms to the DTD; a document that conforms is called valid.

The data passed into and out of a 4GL service call procedure consists of a list of parameters that can be simple scalars or objects and, hence, can be hierarchical in nature. By mapping the parameter structure into a related XML document structure, you can send and retrieve parameter data as XML documents. The 4GL parameter names are represented as XML entity names, and 4GL parameter values are represented as XML entity values. When requested (in the client call) XML-to-4GL mapping occurs automatically. It is available for method GSCPs and for procedure GSCPs. It is not available for regular user-written 4GL SCPs.