What's the difference between spoil and fill?
Spoil is the material excavated out of the ground. Fill is material placed and compacted to build grade back up. Spoil can sometimes be reused as fill, but it swells 20 to 30 percent when dug and never all fits back in the hole.
Undisturbed soil is dense. The bucket breaks it up, air gets between the particles, and one bank yard becomes 1.2 to 1.3 loose yards on the pile. That swell is why the pile always looks too big, and why hauling is priced on loose yards while the hole is measured in bank yards.
When spoil can go back
Clean native soil without organics, debris, or excessive moisture can usually be compacted back as fill for non-structural areas. Around pipe, under slabs, or under anything that settles expensively, specs typically call for select fill: screened material or imported granular fill that compacts predictably.
The math that bites
Even reusing every yard of spoil, the pipe, bedding stone, and structure you installed displaced volume, so there's always a surplus to lose on site or haul away. Figure that haul-off into the bid, not into an argument at the end of the job.