When some (or all) of the items on an Order are shipped, the Order and the new Invoice have the same transaction number. Both the Order and the Invoice have a letter after the transaction number.
The following example is for an Order with the transaction number 123. If:
Nothing Shipped, there is no Invoice, Sales Order 123 is created.
Everything Shipped, there is no Order, Sales Invoice 123A is created.
Partial Shipment, both Order and Invoice are created, both transactions are 123A.
Second Partial Shipment, Order is updated and new Invoice is created, both transactions are 123B.
In the above example, the First Order is still 123A. Each new Order/Invoice will be lettered consecutively.
See:
Sales Manager Contents