Kerf
The width of material removed by a cutting process, which sets the spacing between nested parts and quietly reduces the yield on every sheet.
Kerf is the material the cutting process consumes as it goes. A laser, a plasma torch, a waterjet and a saw all remove a measurable width, and that width has to sit between every pair of adjacent parts on a sheet.
On a single part it is invisible. Across a nest of 84 blanks it is a meaningful share of the sheet, and it is one of the reasons the yield on a real layout never reaches the theoretical figure somebody calculated from part areas.
It also affects the part itself rather than only the spacing. A profile cut on the line rather than on the correct side of it comes out undersize by half a kerf all round, which matters on a part with a tight fit and matters not at all on a bracket.
For estimating purposes the practical consequence is that the web between parts is not a matter of preference. It has a floor set by the process, and a nest drawn tighter than that floor is a nest that will not cut.