Volume for the pour you are actually doing, with the bag count worked out. Ready-mix is sold by volume and bags are sold by weight, so the two numbers are given side by side.
Notes
Why order more than the calculated volume
Subgrade is never perfectly flat and forms bow outward under the weight. 5-10% over is standard, and running short mid-pour leaves a cold joint you cannot undo.
Bags or ready-mix?
Bags stop making sense somewhere around half a cubic metre. Past that you are mixing for hours and the first batch is setting before the last is poured.
Post holes
The hole volume minus the post volume is what actually gets filled. For several identical holes this adds up quickly, so the tool subtracts the post.
What this does not do
It does not specify a concrete mix, reinforcement or thickness. Those depend on load and ground conditions and are not a calculator question.