I believe AttributeSelector uses XPATH, and AttributeDesignator uses our
own home grown "selection" into the context.
From reading through the schema and browsing the email archive it looks
the old designator with an xpath expression has been split into selector
is the xpath and designator which is just the name of the attribute. I'm
what the attributeid of the designator is though.... or which attributes
to designante... it could be limited to those defined in the context...
selector is for selecting attributes in the resource. Anyway I'll just
guess for now and wait for the next draft to be completed.