I asked a similar question about a year ago, in a thread entitled Working with XML objects. From that thread, here is my understanding of the function of the XML_* script elements:
Script elements for working with structured documentation tabs
The AE scripting language includes many functions for working with structured documentation tabs, including:
Using these scripting functions, one can easily manipulate individual parts of the XML.
These commands are not capable of opening, reading, manipulating, or writing XML files or XML VARA objects. They are only capable of working with XML DOCU tabs.