In procurement drawings, quality reports, or surface treatment requirements for precision-cast and CNC-machined parts, terms such as Ra, Ry, Rz, and polishing levels are frequently seen. While all related to surface finish, they are not the same concept. Choosing the wrong indicator may result in parts that, although "appear shiny," fail in sealing, fit, wear resistance, or coating adhesion. This article, starting from the practical applications of stainless steel precision casting and CNC machining, compares surface roughness indicators, common polishing conditions, metal wear risks, and subsequent surface treatment selection strategies. I. Surface Finish…