ACR Values are optional parameters, provided as a space-separated string. These values specify additional context values which the authorization server shall use to process the client request for authentication. If the client provides a value the IdP understands, it will be respected but if the IdP does not understand it, the authentication request is either denied or the acr_values are sent back to the client, which can then decide if the level of authentication is satisfying and carry on with the authentication or reject it. The values appear in order of preference. Possible values may be multi-factor or phishing-resistant. See here for more information: