Why "Google Play Games sign-in" authCode is the string "Success"

I use Google Play Games sign-in api:

Social.localUser.Authenticate( (success,authCode) => {
                if (success)
                {
                    debugger.Log($"Login with Google Play Games done. authCode: {authCode}, success:{success}");

the success is true, but the authCode is the string :"Success".

8447414--1120274--upload_2022-9-18_14-37-54.png
8447414--1120277--upload_2022-9-18_14-38-11.png

I found when I use "PlayGamesPlatform.Instance.RequestServerSideAccess", it works.

Social.localUser.Authenticate( (success,authCode) => {
                if (success)
                {
                    debugger.Log($"Login with Google Play Games done. authCode: {authCode}, success:{success}");
                    PlayGamesPlatform.Instance.RequestServerSideAccess(true, async code => {
                        debugger.Log($"PlayGamesPlatform.Instance.RequestServerSideAccess. code: {code}");
                        await AuthenticationService.Instance.SignInWithGooglePlayGamesAsync(code);