XML Schema: Substitution Groups

Question

I am studying lookup groups in an XML schema, and I cannot find the official specification. can anyone point me to this?

I am wondering if an element with a substitution group attribute can replace an element inside complex or only global elements like w3schools :

"Please note that all elements in the substitutionGroup (head element and replaceable elements) must be declared global elements, otherwise this will not work!"

Is this statement above?

since global elements can be the root of an XML instance - what is the point of defining substitution groups on them?

Answer 1

- . , , , <xs:choice> . , , .

, , , , . (. , ).