Each unit can have custom tax rules, so if you need to charge a hotel tax on reservations less than 30 days, local tax on all reservations, and sales tax on reservations 30 days long and longer for some units while other units are not taxed at all, that is all possible.
Set up your taxes
First, you need to set up your tax names and rates. Click on Settings > Taxes to view all your current taxes. To add a tax, just enter the name of the tax and the rate and click Add.

Once you have added all your possible taxes, you are ready to set up the taxes for each unit.
Adding a new tax rule
Go to Settings > Units, and select a unit you want to set up rules for taxes. Click on the Taxes tab, then click Add Tax Rule.

Select the tax you want to add to this unit, and indicate if the tax should only be applied to the reservation depending on how long the reservation is. So for example, if you only want a tax to be applied if the reservation is 30 days long or less, set the Max Stay Length to 30 days.
Once you have selected the tax and set the min/max stay length restrictions, click Save.

You will now see all your tax rules listed for that unit. If you want to copy these same tax rules to any other units, you can export the tax rules using the import/export unit info feature.
Override taxes on a charge
If you ever need to manually modify which taxes are applied to a specific charge, you can always edit the charge taxes from the edit charge dialog. So navigate to a reservation, open the options menu next to a charge on the charges tab, and click Edit to edit the charge.

Next check the checkbox for each tax that should be applied to this charge.

After you have selected all the taxes you want to apply, click out of the tax box to close the list of taxes. The tax due total is automatically recalculated to include only the taxes you just selected. Click Save, and the charge will the saved with the new taxes!