A stacked order is when you are already on an order and another request comes in, typically says "add order to route".
A multi-stop order is when the request comes in with 2+ orders already attached to it, it will notify you next to the store name.
The drivers utility helper app will also attempt to notify you of this as well when the overlay shows up by stating multi-stop order.
Currently the Drivers Utility Helper app does not auto accept or decline multi-stop orders, so you will have to manually make a decision for these type of offers.