Exclusive (Add Tax): You have the price of the item and you want to know what the tax will be and what the total cost will be.

Inclusive (Remove Tax): You have the total amount paid (which already includes tax) and you want to know how much of that was the base price and how much was tax.

Remember that many states in the US also allow local municipalities (cities and counties) to add their own sales tax on top of the state rate. Always check your local jurisdiction for the most accurate calculation.