It overrides the state's default tax rate for orders delivered to this zip code. Leave it at -1 (the default) to inherit the state's Tax % – the list then shows -1 (state%). Enter 0 for a real zero-rate zip, or any other percentage to charge that rate. Only -1 (or lower) inherits the state rate; 0 and any positive value are used as-is, so 0 is a genuine zero-rate override, not "inherit".