When Chrome OS device is enterprise enrolled, organization admins can add two special types of users on the device. Those are Public sessions and Kiosk apps.
Public session can be described as managed guest session: guest mode that is controlled by user policy.
It does not have any real google account, it is ephemeral (no data would be persisted on the device after session is ended), but organization admins still have some control over that: they can pre-install extensions, set policies, install certificates.
To set up public session, go to Admin console under Device Management > Chrome Management > Device Settings, Kiosk settings section, and set Public session kiosk setting to Allow. You can also configure additional settings.
Kiosk mode is a session that runs a single Chrome/Android app.
It does not have any real google account, it is persistent (data will be persisted between kiosk sessions) by default.
Multiple kiosk apps are allowed per device, and they can be launched from system shelf on the login screen. Additionally, the administrator can set up one app to launch automatically on start-up, in which case login screen is not shown unless the user cancels the app launch from the app launch splash screen.
To set up kiosk mode, go to Admin console under Device Management > Chrome Management > Device Settings, Kiosk settings section, add available apps under Kiosk apps / Manage Kiosk Applications.
You can configure kiosk app settings under Device Management > Chrome Management > Apps & Extensions