Variable definition and assignment question

Hi all,

   In a Biztalk 2004 orchestration, I have a message of type XML schema 
(linked to an XSD file) and I want to assign that message to a variable in
an 
expression shape.
   My questions are: 
1) What type should I define for that variable?
2) What is the syntax for assigning the message to that variable in the 
expression shape?

   Thanks for your help!

    Daniel.