Passing Primary Document to Child Business Process

Using the below assign statement we can pass PrimayDocument and other required value from process data to chaild business process which is going to be executed in Async,

<assign to="message_to_child" from="PrimaryDocument | /ProcessData/Xpath1/text() | /ProcessData/Xpath2/text()"/>

<operation>
    Invoke Business process in Async mode from here.
</operation>