Validation of XML inputs
The XML validation options enable XML schema processing in the XML parser. This performs a basic validation of the input and is highly recommended especially for beginners because it easily finds spelling mistakes in the input which otherwise might be silently ignored. Be sure that your input files have a header like
<routes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="http://sumo.dlr.de/xsd/routes_file.xsd">
because only if the schema is mentioned explicitly validation will occur (unless you enforce it using the "always" mode). For details see XMLValidation
Schema validation may slowdown XML parsing considerably and is therefore disabled for the network input by default (because networks should not be edited by hand and therefore be valid anyway). If you have large auto-generated inputs you may consider disabling schema validation altogether. See also this FAQ.