How does a welder know what electrode he is using?

Almost all electrodes have a series of numbers to tell #1 what the tensile strength of the weld will be, #2 what electrical current to use, #3 what position/s it can be used in (ie: flat, vertical etc.) Ex. E - 7018. E = electrode, 70 = 70,000 lbs tensile strength, 1 = all positions, 18 = AC or DC reverse polarity current. (welding lead +)