If you cross the U.S. border into a foreign country and reenter the United States more than once in a short time, you may not want to use your personal exemption until you have returned to the United States for the last time. This is due to the "once every 30 days rule"-you can only apply your personal exemption once every 30 days.
So as an example, you go to Canada, buy a liter of liquor, reenter the United States, then go back to Canada and buy $900 worth of merchandise and more liquor. You would probably want to save your $800 exemption for those final purchases and not use it for that first liter of liquor. In this case, on your first swing-back, simply tell the CBP officer that you want to pay duty on the liquor, even though you could bring it in duty free.