At run time, instances of XML document types and XML fields are of contained in an XMLData object. XMLData is an IData object that uses a specific encoding format to represent the XML Information Set (XML Infoset). The format facilitates all the features of XML Infoset and XML Schema, including support for capabilities such as nested model groups and substitution groups. The format also eliminates the need to specify an association between prefixes and namespace URIs.

Unlike the traditional encoding used for XML representation with raw IData, the encoding format for XMLData is not public and is subject to change at any time. The various built-in services that support XMLData, including those in the pub.xmldata folder in the WmPublic package, and flow MAP step operations are the only supported means of accessing and modifying XMLData. Directly manipulating an XMLData object as one can a traditional IData object will lead to unexpected results.