The Sigma Level is a widely used metric in Six Sigma, also known as Z-Level or Z-Score. It is used to assess process capability and indicates expected defect rates and areas for improvement. The Sigma Level represents the number of standard deviations a process can fit between the process mean and the tolerance limits, assuming all tolerance violations are one-sided. It is derived from calculating the Z-value, which indicates the distance of the process mean of normally distributed process outputs from a given tolerance limit, and by determining the proportions of under- and over-shoots using the Z-value table.