Dereferencing a Reference with no URI attribute

I have a question about dereferencing (or identifying) 
a Reference without a URI attribute. Section 4.3.3.1 of 
xmldsig-core states, 4th paragraph:

"If the URI attribute is omitted altogether, the receiving 
application is expected to know the identity of the object".

Further on, in section 4.3.3.2, it states:

"Unless the URI-Reference is a 'same-document' reference as defined 
in [URI, Section 4.2], the result of dereferencing the URI-Reference 
MUST be an octet stream."

Does the statement above apply to a Reference with no URI 
attribute? Can it be represented as either an octet stream or 
a node set? Or, since it is undefined, is it technically NOT a 
same-document reference, and therefore MUST be dereferenced/identified 
as an octet stream?

Thanks,
Sean