I think what you're talking about is a random number that is used to associate advertisement impressions to advertisement clicks.
Here's a typical ad that uses that format.
<a href="http://adnetwork.tld/click?ad=1234567&session=98765">
<img src="http://adnetwork.tld/view?ad=1234567&session=98765" />
</a>
Here's a typical way of generating that ad on the client side with Javascript.
<script language="javascript" type="text/javascript">
var random = Math.floor(Math.random() * 100000000);
document.write('<a href="http://adnetwork.tld/click?ad=1234567&session=' + random + '">');
document.write('<img src="http://adnetwork.tld/view?ad=1234567&session=' + random + '" />');
document.write('</a>');
</script>
I'm sorry if that wasn't what you were looking for but that's what it seems like going off of your question.