no, thats not correct.
It wont work to use different shipping services for each item in the cart, but that is not a common use case scenario.
the more common use case scenario is to use the same shipping service for all items in the cart.
The shipping lookup work in a couple different ways:
1) Ship each item in a separate box. using this option, it calculates each item individually by item weight for the selected shipping service.
2) Ship the entire order in one box. This option calculates the shipping cost based on the total weight of all the items in the cart for the selected shipping service.