what is the concept of risk events ?

Example:

The Microsoft documentation classifies the risk event of "signs in from an unfamiliar location" as of the risk level of "Medium"

reference:

https://docs.microsoft.com/en-us/azure/active-directory/identity-protection/overview-identity-protection

classifies the risk event of "signs in from an anonymous IP address" means what is the risk level ? = Medium 

Example02:

classifies the risk event of  "signs in from infected devices" means what is the risk level ? = low