High-Speed Steel (HSS) taps are primarily categorized by their alloy composition. For general-purpose machining, HSS-E (Cobalt-bearing steel like M35 or M42) is highly recommended as it offers superior heat resistance and hardness, making it ideal for stainless steel and tough alloys. For standard carbon steel, HSS-M2 is the industry benchmark for balancing toughness, wear resistance, and cost-efficiency. Always verify the Rockwell Hardness (HRC), which should typically fall between 63-66 HRC for quality tools.