Preferences - Taxes
Keep all Debt/Credits updated with default Tax types - When editing a Debt/Credit type, all Debt/Credits of that type will receive that tax information of that Debt/Credit type. Warning: This will overwrite and manual changes you've made to a Debt/Credit's tax settings.

Edit Tax Window Options

Name - Name of the tax.
Percent - Percent of the tax.
Mode - Standard is a regular tax. Piggybacked taxes are applied to the total of another tax. For example, if Tax A (10%) is standard and Tax B (7%) is piggy backed onto Tax A, and they are both on an Invoice for $100, Tax A will total $10.00, while Tax B will be 10.00x0.07=$0.70.