In the workshop, the word coating covers at least four different technologies, and confusing them is one of the most common mistakes I hear even from people who've been selling watches for years. The only thing they have in common is that they modify a component's surface without changing its geometry: everything else — process, hardness, thickness, color — changes radically from case to case.
PVD (Physical Vapor Deposition) is the treatment I encounter most often. The coating material — titanium nitride, chromium nitride, titanium carbide depending on the desired color — is vaporized under vacuum and deposits on steel in a layer just a few microns thick. The result can be matte black, industrial gray, rose gold, yellow gold: it depends on the compound used. Hardness generally reaches 1,000–1,200 HV on the Vickers scale, compared to 150–200 HV for raw 316L steel. It's not electroplating in the galvanic sense: the layer is thin but much harder, and doesn't crumble like electroplated gold can. It does wear at friction points, something anyone buying a PVD watch should understand from day one.
DLC (Diamond-Like Carbon) pushes hardness even higher, up to 3,000 HV in certain processes. Structurally it's amorphous carbon with diamond-like characteristics: black, almost frictionless, practically immune to ordinary scratches. It's used mainly on sports cases where PVD isn't enough. CVD (Chemical Vapor Deposition) works through gas-phase chemical reactions rather than physical deposition: it allows colored finishes — blue on hands and indices is the most common example — with a color depth that PVD rarely achieves.
The AR (anti-reflective) coating on sapphire crystal is a different category altogether. It consists of extremely thin layers of metal oxides stacked so that reflections cancel out through optical interference. Applying it only to the crystal's inner surface protects the coating from wear but reduces reflections less effectively than double-sided application; applying it to both surfaces maximizes readability but exposes the outer layer to scratches from cleaning. Ten AR layers, as I've read on certain sapphire crystals, isn't marketing hype: each layer covers a different band of light frequency.
Luminescence — Super-LumiNova and its derivatives — is technically also a coating, though rarely called that: a layer of photoluminescent material applied to hands and indices. Designations like X1 BL C1 indicate the formulation and the tone of the dark emission, not the generic quality of the product.