Turns a triangle mesh into a STEP solid that CAD programs can open. Every vertex is written to the file exactly as it appears in the STL.
Each STL corner point is written with every digit it holds, so the STEP vertices match the STL to zero error. Lines and planes are checked against those points after writing.
Merge flat areas joins triangles on the same plane into one face. Round shapes turns holes, shafts, fillets and chamfers into true cylinder and cone faces with circular edges, so CAD loads them fast and can measure them. Simplify the rest thins out the triangles that are left, keeping every remaining point exactly where the STL had it.
A watertight mesh becomes a solid body. A mesh with holes is written as a surface body unless you fill them. Fill holes closes each gap with the smallest patch that fits its edge, without moving any existing point.
STEP is much bigger than STL: expect roughly 1 KB per triangle. Meshes over about a million triangles may run out of browser memory.