priority attribute question

I see the barx resolver returns me an XRD with a priority
attribute on the CanonicalID element. 

 

I was a bit surprised to see that since section 4.3.3 of WD11/ED
06 doesn’t mention CanonicalID as an element that can have a priority
attribute and neither does section 4.2.3. I *do*
notice that the XSD schema from WD11/ED 06 allows it. 

 

So which is it? What elements can have the priority
attribute? 

 

            -Gabe