This Ash dialog is for authenticating the user during a user session. The primary use case is WebAuthn, where a platform daemon (u2fd) receives an authentication request from the Chrome browser, and needs to initiate a user authentication dialog which could involve fingerprint, PIN and password. More authentication methods, such as SmartLock and smart cards, might be added in the future.
This dialog is designed to be reused by other projects that need to trigger in-session user authentication from ChromeOS, such as authenticating for ARC apps.
This dialog is controlled by ash::AuthDialogController. When the user provides a credential, the controller talks to cryptohome via chromeos::ExtendedAuthenticator for authentication.