Surface treatment is often discussed late, after the machined geometry is already quoted. That can create cost and timing changes if the coating affects thread fit, bore size or corrosion requirement.
Plating, passivation, anodizing and black oxide all serve different purposes. Buyers should explain whether the goal is corrosion resistance, appearance, conductivity, wear behavior or assembly compatibility.
Weeda Precision reviews surface treatment together with material and tolerance. For small precision parts, the final coating can be part of the functional dimension, not only a cosmetic detail.