HKS is a German color system, which was developed by the companies Horstmann-Steinberg, Kast + Ehinger and H. Schmincke and Co., hence the name: HKS. It is quite common in Germany, but plays nor role internationally and is increasingly driven by Pantone color as a global system to the brink.
HKS is available in four different color guides:
HKS K for coated paper
HKS N for uncoated paper
HKS Z for newsprint
HKS E for rotogravure
The original 88 colors were a few years ago expanded with the new HKS 3000+ color fan to 3520 colors that do not really have different colors as opposed to Pantone, but have more shades through overprinting the actual colors with black. By doing so, 39 shades per color are generated.