Identifies this login attempt. In multi-step methods, send it back in the next call, in the authentication_attempt_identifier field of the request's union member. Ignore it for AUTH_DATA_TYPE_CREDENTIALS.
Used by methods
IRPCLogin.LogIn -- Authenticates the caller and opens a session to a vault, or to the server when the vault GUID is all zeros.
LogInResponse.authentication_data -- The server's half of the authentication exchange. For multi-step methods it carries the next challenge, whether the login is done, and the attempt identifier to send back. For AUTH_DATA_TYPE_CREDENTIALS there is nothing to act on.