This line specifies the linear relationship between the risk $(\backslash$sigma M

$)$ and the expected return $($r$$P

$)$ for efficient portfolios.

The dependent variable is the expected return on an efficient portfolio, and the independent variable is the risk represented by an efficient portfolio in addition to the risk$-$free rate.

This line specifies the linear relationship between the risk $($bi

$)$ and the return $($ri

$)$ for an individual security or a portfolio.