In sales, shipping you often have a lot of variation in working with customers some want partial shipments, others only want full shipments. Sometimes you might not have full stock to fulfill a shipment so you only send partial. Sometimes you have lots of orders so you have to balances across a few customers to allocate the stock you have and ship partial and follow-up with another shipment to complete the order.
If you are using the advanced warehouse then co-ordinating between sales, warehouse and accounts receivable team is important to co-ordinate order, ship, invoice.
Here is a quick walk through of a basic partial ship, and invoice flow in AX2012 R3