If this policy is set to true or not configured, Google Chrome OS will show existing users on the login screen and allow to pick one.
If this policy is set to false, Google Chrome OS will not show existing users on the login screen. The normal sign-in screen (prompting for the user email and password or phone) or the SAML interstital screen (if enabled via the LoginAuthenticationBehavior policy) will be shown, unless a Public Session is configured. When a Public Session is configured, only the Public Session accounts will be shown, allowing to pick one of them.
Note that this policy does not affect whether the device keeps or discards the local user data.