From the rate on the bag
Each step has a Rate, in lb per 1,000 sq ft or in g/m², a Bag size and a Spreader setting, all copied from the bag. The app works out the pounds for each zone, then the bags under To buy.
Counted on the whole lawn
Bags are counted on the total, not zone by zone: a bag opened in front is finished in back. Per zone gives one zone its own rate, for shade or overseeding — Rate for this zone, empty to use the step's rate.
Your products
The presets are common label rates, not recommendations. Lawn Calc never tells you which product to buy: the rate, the brand and the setting are the ones printed on your bag.