State uses XML in quite a few places. With mod_xml, State will dump XML files containing useful health related information that other tools can use to display information. As an example, mod_xml will write a nodes.xml file whenever node health changes containing the status of all network nodes. The mapping system then uses this file to determine which nodes are up and which are down. Third party tools that dump Vital tags also use mod_xml, as it's where that tag gets turned into an actual vital update.