What does CS or HS mean in thread cutting tools?

CS refers to Carbon steel where the main alloying constituent is Carbon. It has low or trace amount of other alloying constituents. CS is cheap and has good strength and hardness and can be used for basic tooling applications.

HS or HSS refers to High-speed steel where alloying constituents other than Carbon such as Tungsten, Molybdenum, Cobalt and Chromium are used in higher amounts. HS is more expensive and can withstand higher temperatures without losing its hardness. This enables HS to be used in high-speed tooling applications.