Tip amount, total bill, and the split per person.
The calculator applies the tip percentage to whatever number you enter in "Bill amount" — it doesn't separate tax from the subtotal. If you want the tip on the pre-tax amount only, enter the pre-tax subtotal rather than the receipt total.
The total (bill + tip) is divided evenly by the number of people entered in "Split between" — it's a straight even split, not based on who ordered what.
That section recalculates the tip amount and per-person share at 15%, 18% and 20% using your entered bill and people count, so you can compare a few standard tip levels side by side without re-entering numbers.