In AU orders, the tax rate/tax code can be different in order line level. Which means in a particular order order lines can have different tax code/rate. And this tax code/rate is fixed for the whole country.

But when it comes to US, The tax code is different for each state. And the tax code is fixed for the one particular order where it can not be changed in line level. Since the tax rates are different for each states, it is hard to maintain the tax code and rate to decide which one needs to be picked for the order.

Therefor we use the retailer setting, Use Customer Tax Rate in Supplier Retailer Settings. The user needs to specify the tax code and rate for each customer in different states. When the order validates the system picks up the customer tax code and rate and import that value.