This category includes XML Schema Definition Documents (XSD's) related to data documents or application resources. For example, the 2004 Schema definition for the IGS Site Log is actually a compound definition relying on lower-level base classes. The purpose of a schema definition document can be to produce an actual data file in XML format, containing data, or it can be to define information used by Web Services applications. Web Services uses small XML documents to communicate. SOPAC anticipates a compound IGS Site Log document, made up of multiple base classes, and a variety of smaller XML definitions related to the IGS Site Log supporting Web Services.

The 2004 version of the IGS Site Log is anticipated to be the final release developed by SOPAC before it is open to review by the public. SOPAC gained experience developing the original 2002 XML Schema Definition document that was a single document, not relying on sub-classes. The 2003 version used sub-classes, but used a single namespace. The 2004 version uses sub-classes (base classes) and allows them to come from different namespaces. This will allow different organizations the ability to seamlessly maintain sub-class documents. SOPAC is using Schema (XSDL) as the schema definition language.