How deep a steel can be hardened, not how hard it can become.
Hardenability is about depth, and it is governed by alloy content — chromium, nickel, molybdenum and manganese all increase it. A steel with high hardenability can be through-hardened in a large section; one with low hardenability may only harden a surface skin.
It is frequently confused with hardness, which is how hard the surface becomes and is governed by carbon content. A 0.2% carbon steel can have excellent hardenability and still only reach 45 HRC.
Reading a data sheet's hardenability figures as hardness figures. One describes depth, the other surface hardness.
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