xml schema Second Edition-regarding substitution group

I just read the Xml Schema Second Edition .In the section of substitution group it has been said that both the head element and the substitutable elements must be declared global.
If the substitutable element happens to appear in the content model of a complexType definition then while deriving a new complex type by restriction,this element could not be restricted (by its type) if it is declared globally.
hence there should be a possibility to declare the substitutable element locally also.
any comments regarding this.
 
regards,
V.Gayathri.
 
 
 

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