Clarification: use of Schema namespace aliases within URLs

CSDL spec section 3.4.2 states:

"An alias is only valid within the document in which it is declared; a referencing document has to define its own aliases with the edmx:Include element."

Protocol spec section "11.2.4.1 System Query Option $select" states:

"The value of the $select query option is a comma-separated list of properties, qualified action names, qualified function names, the star operator , or the star operator prefixed with the namespace or alias of the schema in order to specify all operations defined in the schema."

So now we see that aliases can be valid/referenced outside their defining document.

If a URL uses an alias, is it limited to using an alias defined in the outermost CSDL document, i.e. it a URL prohibited from using aliases defined within referenced CSDL documents? If it is not so limited, then how do we resolve ambiguity of potentially multiple-defined aliases (defined with two different meanings in referncing and referenced CSDL documents)?