This function is only valid when exporting using the UWP target for the Xbox One and it requires you to have checked the Enable XBox Live option in the UWP Game Options. The function will check to see if a user is in the process of signing-in and return true if that is the case and false otherwise.
global.GamerTag = xboxlive_gamertag_for_user();
The above code checks to see if a user is signed-in to XBox Live and if they are signed-in, the code will get the gamertag for the user and store it in a global variable for future use. If they are not signed-in then the code checks to see if they are in the process of signing-in, in which case nothing further will happen, and if they are not, then it will open the account picker, prompting them to sign-in.