The rebar is made of hot rolled steel. Most steel bars are produced from steel ingots, while the rest are from steel sheet edges or railroads. The rebar needs some sort of identifier that can be used to identify them in rolling mills.
The ASTM has developed a standard identifier that all longitudinal sections must comply with: The numbers represent the size of the rebar. Types of steel symbols eg. W for welding steel (ASTM A-706) or A for axis (M 322 / ASTM 966), etc., indicate that the rebar is rolled up of new ingots. Rebar identifying grades 60 or 75, or 420 or 520 are metric measures, indicate the yield strength.