How to Setup Tax Explanation Code for Customer for different Provinces (States)

I have an error on a screen that says "When a tax or taxable amount is entered, a tax explanation code is required. Codes are validated against UDC 00/EX." This error is thrown by X00TAX.c which I think is called from the F0911EditLine.

So a Tax Explanation Code can be entered into the Address Book and/or Customer Master. But what would it be when a Customer is located in one Province/State and they are buying in different Provinces/States where the Tax Charged is different in each Province/State. In this case, the Customer is located in Ontario and buying in British Columbia, and therefore needs to pay GST Tax.