What is the difference between essential and supplementary essential variables?

Essential variables are mandatory.  Supplementary essential variables are those variables that must be considered when notch toughness requirements are invoked. You must first review the applicable welding standard to determine which welding parameters are defined as essential, nonessential, and supplementary essential variables. Welding documents have to properly address the three types of variables in order to serve as the foundation of a successful welding program.  Heat input is a factor in determining notch toughness. Voltage, amperage, and travel speed are important considerations in  determining heat input, within specified limits.