A key reference defines a set of values that must match some key in the document.

Each field must match, in the same order, to the fields of the refered key.

The 'refer' attribute points to a key via a QName.

A key reference specified by the 'keyref' element with:

• a required 'name' attribute.

• a required 'refer' attribute that names a matching key.

• One 'selector' child.

• One or more 'field' children.

A keyref declaration:

<xs:keyref name="product-reference" refer="my:product-name">
<xs:selector xpath="."/>
<xs:field xpath="product-used"/>
</xs:keyref>