policy reference file about element

I understand that the 'about' element in the prf may be an absolute or relative URI. I thought I remembered that an absolute URI had to point to a policy file in the same top-level domain as the one where the prf is located, but when I went back to the working draft to double check, I didn't see anything about it. 

What if any is the restriction on the URI in the prf about element?

tia.