I think Stefano's comments intersect with some of my suggestions. See:

http://lists.w3.org/Archives/Public/www-rdf-dspace/2004Jun/0001.html

I think key structures like relatedItem and ref ought not to be such
generic elements, or at least as he says, that the type attribute value
in particular on relatedItem ought to be required. It doesn't make any
sense to specify something as related without saying what the nature of
the relation is.

Bruce