What is the difference between the security market line and the security characteristic line?

Security characteristic line (SCL) is a regression line, plotting performance of a particular security or portfolio against that of the market portfolio at every point in time. The SCL is plotted on a graph where the Y-axis is the excess return on a security over the risk-free return and the X-axis is the excess return of the market in general. The slope of the SCL is the security's beta, and the intercept is its alpha.

The Securities Markets Line is the line produced by CAPM, so the y-axis is the required return per CAPM and the x-axis is beta.