Is XML Schema a constraint language or a specification language?

Hi Folks,

Is an XML Schema a constraint language? An XML Schema states constraints that must be satisfied by instances for them to be "valid". Determining if an instance is valid is a constraint satisfaction problem. 

Or, is an XML Schema a specification language? An XML Schema is a specification that informs instance document authors how to construct instances. Determining if an instance is valid is a matter of checking that the instance adheres to the specification.

/Roger