Topsoil is the upper, organic-rich layer of soil. It's unsuitable for structural fill or pavement subgrade because it compresses, decomposes, and loses bearing capacity over time. Every site that involves grading or paving starts with a topsoil strip.
Strip depth depends on the soil profile and the geotech recommendation. Typical residential and commercial sites strip 4-8 inches; agricultural and forested sites can strip 12-18 inches. Strip area is the full disturbed footprint, including grading limits beyond the building.
Stripped topsoil is bid as a separate line item — stripping (excavation), stockpiling (on-site or off-site haul), and re-spreading (placing on finished landscaped areas at end of project). Each is its own unit price. Don't roll topsoil into the structural cut volume; it has its own scope and pricing.