From this link
SOAP Schema Def (XSD) It appears that mustUnderstand is a global attribute, which means any element inside the Envelope has access to this variable.
If this attribute was restricted to any elements in Header element itself, It would not have be declared as global element.
Harsha
SCJP/SCWCD/IBM XML-Dev/SCEA
[ November 12, 2006: Message edited by: Harsha Jay ]