There are two different things here, and keeping them apart is what makes your reports trustworthy.
Discounts
A discount reduces what the customer pays. On the Discounts screen you build presets as a plain sentence: "Staff meal takes 50 percent off the bill", or "Monsoon offer takes 100 rupees off the bill". Each preset shows as a coupon, and the cashier applies it with one tap instead of typing a number under pressure.
A discount can apply to a single line or to the whole bill. A line discount can never exceed the value of that line, so a mistyped figure cannot push the order below zero.
Free of charge
A free item is not a sale at a price of zero. It is stock that left the shop without money coming in: a staff meal, a replacement for a dish sent back, a sample. Record it under Free of charge so it is deducted from stock and listed as a giveaway, not mixed into your sales figures.
This is the difference between a report that says "we sold 40 plates, 3 of them free" and one that says "we sold 43 plates at an oddly low average price". The first tells you something. The second hides a cost.
Reading it back
Discounts and free items both appear in the cash book for the period you are looking at, so you can see what you gave away in a month and decide whether the offer earned its keep.