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.
For example, if you have a local tax that is rising on Jan 1, 2024 an extra $1 per night:
Your existing $4/night tax (let’s call it Tax2023) needs to have an end date of Dec 31, 2023. Then, make a new Tax (such as Tax2024) as a flat tax of $5 with the start date set to Jan 1 2024. Both taxes need to be allocated to Item: RENT to have arriving bookings quoted prior to Jan 1, 2024 at $4/night (using the Tax2023 setting) and then after Jan 1, 2024 the quote would use the Tax2024 setting as $5/night.
The old Tax2023 will stop being relevant after Jan 1, 2024.
Your OTAs - such as Expedia, Booking, Airbnb, etc - that you are under contract with - need to have similar settings set in their extranets, as BookingCenter doesn’t affect those tax rates.