In large schema files, it is helpful to document parts with comments.
Comments always start with <!-- and end with -->. In between may lie a few comments, but also entire sections.
<!-- This is a comment line -->
<!-- The following objects are disabled by comment
<node id="Valve" name="${Valve_Name} typeClass="${ModularSystem.Function.Valve}"/>
<node id="Cylinder" name="${Cylinder_Name} typeClass="${ModularSystem.Function.Cylinder}"/>
<node id="Sensor" name="${Sensor_Name} typeClass="${ModularSystem.Function.Sensor}"/>
-->