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what is the meaning of Datum?

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Hi all,
ok this is a silly question, but I've got a doubt: what is the exact meaning 
of Datum? I see having a look at the macroes (e.g., PG_RETURN_XXX) that a 
Datum can be used as a pointer or as a single data, that is it can be a 
reference or a value. Is this right? So for instance the fact that each stored 
procedure returns a Datum means that the semantic of the Datum is interpreted 
depending on how the procedure is defined in the SQL language (i.e., it returns 
a integer, a tuple, ...). Am I right?

Moreover, is there a documentation (aside the source code) that explains and 
links each internal data structure like HeapTuple, HeapTupleHeader, and so on?

Thanks,
Luca

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