The optimized plan is then executed, producing a ground expression stream; (a ground expression is a constant expression with no functions or variables).  The ground expression is then serialized or converted into an XML fragment stream.

Type System
XQuery is a statically typed language, which uses type information to statically check queries for correctness.  XQuantum generalizes XQuery's sequence type syntax to include full regular expression types.  The extended type system makes working with XML data types much simpler.

XQuantum types XML content using optional type definitions stored in external type modules called .rtd files.  A type module is the counterpart of a query module and defines the data type for a single XML file.  During query evaluation the validator applies this type information to the XML content, creating a typed ground expression stream.