What is "shva" in Gmail's URL?

What is the following portion of a Gmail URL for?

https://mail.google.com/mail/?**zx**=1efobg68r40co&**shva**=1#inbox

If you change it, nothing happens!!

I know Gmail is not an Open-Source program so we can't trace the code. But every website try to make the URL shorter so they ideally shouldn't add redundant data to the URL. At the same time they don't make any difference nor error if they change.

Edit: I know it's a parameter for a scripting language since I'm a PHP developer but as a developer I don't EVER add a useless parameter and I think it's obvious/primitive sense!

What is U 0 in Gmail URL?

The number simply reflects the order in which you logged in to those accounts. The first account you sign in to will have a "0", the second account will have a "1", and so on.

The acronym stands for "Should have valid authentication" as noted here:

http://googlesystem.blogspot.com/2010/07/gmails-shva-parameter.html

As others have noted, 1 is the default value.