What is "shva" in Gmail's URL?

Question

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!

Answer 1

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.