True if the specified document is opened in a separate document window as a subdocument of a master document. Read-only Boolean
Example
This example determines whether Sales.doc is a subdocument and then displays a message indicating it's status.
If Documents("Sales.doc").IsSubdocument = True Then
MsgBox "Sales.doc is a subdocument."
Else
MsgBox "Sales.doc is not a subdocument."
End If