If you are printing Invoices, there is a spec code E14 called 'Order Release #' that indicates which invoice it is, should there be multiple invoices against the same order.

If you use this for a Picking Slip on an order, this spec code prints the number of invoices that have already been shipped for the order. This means that before shipping the 3, this value will be 0. After shipping the 3 and before the 7, this will show a value of 1.

This is strictly used when creating invoices. If you are choosing to save qty shipped on orders, then this information will not be available to print on specs.