Menu -> Calculation Rules

If you have a webshop connected it will copy the calculation rules of the webshop into the POS.
You will only need to configure to each calculation rule where you would like to see the rule.
If you add a new calculation rule in the POS, it wouldn't sync to the webshop.

If you don't have a webshop connected you will need to add  calculation rules in order to add VAT or TAX.