The capital asset pricing model (CAPM) contends that there is systematic and unsystematic risk for an individual

Question:

The (CAPM) contends that there is systematic and unsystematic risk for an individual security. Which is the relevant risk variable and why is it relevant? Why is the other risk variable not relevant?

Capital Asset Pricing Model
The Capital Asset Pricing Model (CAPM) describes the relationship between systematic risk and expected return for assets, particularly stocks. The CAPM is a model for pricing an individual security or portfolio. For individual securities, we make use of the security market line (SML) and its...