Scope of uniqueness of generate-id()

Hello!

Can someone please clarify the scope of uniqueness of generate-id() 
function?

Spec says:
The generate-id function returns a string that uniquely identifies a given 
node... 
An implementation ... always generates the same identifier for the same 
node and 
that different identifiers are always generated from different nodes.
An implementation is under no obligation to generate the same identifiers 
each time a document is transformed.

Does this mean that values of generate-id() call for two different nodes 
from two different temporary trees created in the course of transformation 
MUST be different?

Thanks.
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Vladimir Nesterovsky