Add mode takes a pre-tax amount and shows the tax and total; extract mode takes a tax-inclusive total and recovers the base and the tax. Several rates on one line (GST + service, VAT + levy) are each applied to the base by default, or compounded on the running total when compound is on; the breakdown per rate is printed. Rounding is half away from zero at the chosen decimals. This is arithmetic only, not tax advice — check the rules that apply to you. Everything runs locally.
yes applies each rate on top of the previous tax.