"PSVI, Element Declaration, processor option"

Hi,

Does "at processor option" for the PSVI (Element Declaration) [1] mean, 
that one of [element declaration] or [nil] must be existent, and one can 
specify "at processor option" which one; or does one specify "at 
processor option" if one of the above items should be created at all?

[1] http://www.w3.org/TR/2004/PER-xmlschema-1-20040318/#sic-elt-decl

Kasimer