The TAC system can set sales tax rates in several different ways.
System Default
Rates can be set as a single rate for all sites when using system settings.
Postal or Zip Code
If the rate is populated in the zip code reference file and it is different than 0 and the default rate, it will be set by zip or postal code
County Code
If you assign a county code to a site and the sales tax rate for the county (as set in the county reference file) is different from the system or zip code rate, then it will override the previous rates. In summary, then you can set sales tax rate for a customer by