An XML Schema Definition documument (XSD) can be composed of multiple schema sub-documents, or base class documents. Base class documents define lower-level entities, such as equipment or monuments. By maintaining smaller sub-documents, they are easier to reuse, easier to maintain, and reduce the chance of name collisions. SOPAC has developed a couple of different base class documents related to the Site Metadata including Equipment and Monument, which are used by the higher-level XSD definitions.