Adding tax vs removing tax
Add tax takes a net (pre-tax) price and adds the tax on top useful for quoting customers. Remove tax starts from a gross (tax-inclusive) total and works out the net price and the tax within it useful for bookkeeping and expenses.
Sales tax vs VAT
In the US, sales tax is added at the checkout and varies by state and city. In the UK, EU and many other countries, VAT (Value Added Tax) is usually already included in the displayed price. This calculator handles both just pick the right mode.