The current patient documents and letters are not integrally linked to a item sale, they are linked to a product. They can be created by this linkage. However latter when referencing the item when need a direct link to the invoice item to ensure consistency when reporting batch number and expiry dates.
To achieve this we need to:-
Update the act.PatientDocument* archetypes to provide a reference to the invoice item.
Change the layoutstrategy to hide the node if it is empty
This will then allow the display of batch numbers and expiry dates using