What is welding electrode e 309 meaning?

E means Electrode,309 is used when joining stainless steel to carbon steel. It will have a dash and number following it -15 or -16. The 1 means all position, the two together 15 or 16 denotes the type of current that can be used (both can be used with AC or DC reverse) and the primary ingredient in the coating. It may also have LC or ELC designation. LC means Low Carbon, ELC means Extra Low Carbon. An example E-309-16ELC