This is an internal XQUERY function and you will probably never need to call it explicitly. It is not a part of library of standard XQUERY 1.0 functions.
This function creates an special object, called "attribute descriptor", which may be used only as argument of create-element() XQUERY function. Every attribute descriptor will force create-element() function to add an attribute with specified name and value into opening tag of the created element. If attribute with such name already exists, its value will be replaced.
Attribute descriptor