XSD 1.1, "ref" attribute on IDC constraints

    XSD 1.1 seems to have introduced, an "ref" attribute on IDC constraints as follows,

<xs:unique
    id = ID
    name = NCName
    ref = QName
    {any attributes with non-schema namespace . . .}>
    Content: (annotation?, (selector, field+)?)
</xs:unique>

and similarly for <xs:key> and <xs:keyref>.

The "ref" attribute as mentioned above, was not available in XSD 1.0.

I'm not able to envision a use case of IDC constraints "ref" attribute as mentioned above. Can anyone please, explain a use case of XSD 1.1 IDC constraints "ref" attribute with an example? I'm looking forward for a small example for this, to understand the concept of "ref" attribute as cited in this mail.

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