I found an
$interim_login variable within the
wp-login.php file, and I'm not sure what it does or what it is. The documentation around the interwebs is pretty sparse.
The same can happen in the theme customizer.
So for example if the user is sitting in the dashboard all day and then gets back on and tries to edit something but their session expired and they are no longer logged in WordPress will call 'interim-login' and allow the user to log back in and continue editing where they left off.
You can study how
and of course in