How is this part of the XSLT specification to be interpreted

I am looking at how to make XSL documents self-documenting, and I fell over section 2.1 in the specification

"An element from the XSLT namespace may have any attribute not from the XSLT namespace, provided that the expanded-name of the attribute has a non-null namespace URI. The presence of such attributes must not change the behavior of XSLT elements and functions defined in this document. Thus, an XSLT processor is always free to ignore such attributes, and must ignore such attributes without giving an error if it does not recognize the namespace URI. Such attributes can provide, for example, unique identifiers, optimization hints, or documentation."

My current experience with XT and Saxon is that they complain fiercely if an incorrect attribute is used.  How is the text above to be interpreted? 

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