IV represents four types of steel, belonging to austenitic stainless steel and duplex stainless steel.
Fe IV type (austenitic, duplex stainless steel): 06Cr19Ni10, 022Cr19Ni10, 06Cr17Ni12Mo2, 022Cr17Ni 12Mo2, 022Cr23Ni5Mo3N, 00Cr25Ni6Mo3CuN, etc.

Ⅲ represents three kinds of steel, belonging to martensitic stainless steel, ferritic stainless steel and chromium molybdenum steel.
FeⅢ type (martensitic stainless steel, ferritic stainless steel): 1Cr5Mo, 0Cr13, 1Cr17, 1Cr9Mo, etc.

II stands for Group II steel, which is a low alloy steel. FeⅡ type (low alloy steel): 16MnR, 12Cr1MoV, 20MnMoD, S240, L245, HP295, etc.