In order for tool steels to be identified by a toolmaker, it is necessary for the toolmaker to understand the method of identification and classification of tool steels. The purpose of gaining this understanding is that there are so many different tool steel manufacturing companies that will produce a very high grade of tool steel and name it a particular brand.
The application steels are characterized by the AISI as follows:

1) High carbon/high chromium: D series (Dimensionally stable steels)High hardenability, secondary hardening steels (2% carbon, 5% chrome)

2) Low alloy special purpose tool: L series. Generally a water quench steels.

3) Mold tool steels: P series. Usually low carbon steels, High chrome with moly and nickel.

4) Hot work tool steels: Chromium Hot Work (0.40% carbon, 5% Chrome (H10 TO H19

5) Hot work tool steels: Hot Work (0.35% carbon, 3% to 5% chrome, 10% to 15% tungsten) (H21 to H26)

6) Hot work tool steels,Hot work steels: Molybdenum Hot work (0.60% carbon) 4% chrome, 1.0% Vanadium, 15%,Tungsten, 5% Moly (H42)

7) Tungsten high-speed steel: 0.80% carbon (nominal), 4% chrome,16.5% (nominal) tungsten, 1%,Vanadium T1 to T15

8) Molybdenum high-speed steel: M series HSS M1 to M36,1% carbon (nominal), 4% chrome,
2%vanadium(nominal),2%tungsten(nominal),7%molybdenum (nominal),Some steels have up to 12% Cobalt

9) Ultra hard high-speed steel: M series M41 to M47,1.10% carbon (nominal),4.0%chrome(nominal),3.25% tungsten (nominal),7.5% molybdenum (nominal),7.75% cobalt (nominal)