What is Sloped Area with Depth?

Hey guys,

Can someone explain to me how this tool works, and if i'm using it for the right purpose

I want to calculate the volume of the engineered backfill (highlighted). is this the right tool because the formula say Area??

as you can see the depth and width varies, which is why i prefer to use area and i can adjust the depths on eight side of the trench

Or do i need a custom formula... simply Area x Depth /2 (45degs) then add the 750mm

Here's the cut and fill plan