Yes - no ship-to code on an order means that only the customer tax status is used because nothing else is available to override it.

Every order has a customer tax status, which defines a tax rate % for each jurisdiction. For example 5% for GST and 7% for PST and 12% for HST, or 0% if exempt for a particular tax. An item tax status is defined on every item or misc charge on an order, with similar rates and exemptions. For each item on an order, tax is calculated for each jursidiction by looking at the rates based on the customer and the item. If they are different, then the lower rate is used.

For example, a customer that is PST exempt orders an item that is fully taxable. GST would be 5% on both the customer and the item statuses, thus 5% GST would be charged. But because the customer rate for PST is 0% (and on the item it is 7%), the lower rate of 0% is used and thus PST is not charged for this item. Every item on an order undergoes all of these tax rate comparisons.

Setting up tax tables can be tricky. It is a matter of determining all possible ways a customer can be taxed or exempted and create customer tax statuses and associated rate. And a similar process for items.