Miles by state come from closed trips that carry a miles-by-state split; gallons by state from approved fuel receipts that carry a state and a gallon count. Tax rates change every quarter and are applied on the return itself — this is the worksheet the return is filled from, not a tax calculation. Today the split and the fuel state are entered in the mobile app, so a quarter logged entirely from the web will read empty here.
Net = taxable gallons (miles ÷ fleet MPG) minus tax-paid gallons bought in that state. Positive means tax is owed there; negative is a credit. Apply each state's rate for the quarter on the return.
An estimate from the quarter’s published IFTA rates, before surcharges, interest and penalties. Your base state’s return is the official figure.