Additional Taxes should be used only if the 2 default taxes are not sufficient for your operations. Each additional tax can be either a percent or a flat amount. Use as many taxes as needed.
Additional taxes can be added with or without START and END dates to enable 'date-based' tax calculations. Thus, if you need a tax to only be used for a specific time period, make sure the dates are set correctly and the PMS and Booking Engines will use that tax to calculate tax projections during those times if the tax is allocated for the special RENT item (ie, used to charge nightly Room tax). If a tax is not set for RENT, then the date-based function will only work for Item(s) that are set to use that tax.