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!

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.

If I'm remembering correctly, back when they were working on the current version of the interface, you could preview it by setting shva=2 instead of the default. That version is now the default and you can't get the old version, so shva does nothing now.

It may be used again in the future, who knows?

But every website try to make the URL shorter so they ideally shouldn't add redundant data to the URL...

This is self-evidently not true. Look at StackOverflow URLs for a perfect example. This post:

http://stackoverflow.com/questions/1692968/shva-in-gmails-url-what-is-this

could just as easily be (it works):

http://stackoverflow.com/questions/1692968

I don't think anyone worries these days about the extra couple bytes of data involved with an extra query string parameter.