The term 'hardness symbol' is often used to refer to the STEEL designation system, which is a five-letter code that indicates the type of STEEL used in a particular application. The code is based on the chemical composition of the STEEL and is used to identify the strength and hardness of the material. The most common STEEL designation system is the SAE/AISI system, which stands for Society of Automotive Engineers/American Iron and STEEL Institute.