MATSim generally uses XML files to store data and configuration. There are separate files for configuration, network, demand, and other data. All the different file formats are based on either DTDs (Document Type Descriptions) or XML Schemas, which can be used to validate the XML-files against. The DTD-files and XML schemas are available at http://matsim.org/files/dtd/ and usually contain some documentation about the meaning of the XML-tags in comments.