What is the unique attribute for every single instances of a ASP.net-powered website running side-by-side in different tabs of a browser?

What is the unique attribute for every single instances of a ASP.net-powered website running side-by-side in different tabs of a browser?

What is the unique attribute for every single instances of a ASP.net-powered website running side-by-side in different tabs of a browser? I suspect it is Session ID.
Please guide me.