1. xudml1.xsd file contains the grammar for the structure of repository file:
An XML schema is a description of an XML document in terms of the constraint, content and the structure of an XML document. An example would be DTD (Document type definition), XSD. XSD is also known as XML Schema.
Well formed versus Valid
All xml documents must be well formed however it's not mandatory that they are valid as well. An xml document will be valid if it conforms to its schema definition.
commonly used terms while describing xml docs: tags, attribute, element.
Information about xml schema elements: w3schools
2. biserverxmlgen generates XML from an existing RPD
Before running the XML utilities, you must first run bi-init.cmd (or bi-init.sh on UNIX) to launch a command prompt or shell window that is initialized to your Oracle instance. You can find this utility in:
biserverxmlgen -R D:\xml_api\PS1_14Sept2012.rpd -P Admin123 -O D:\xml_api\PS1_14Sept2012.xml -8