Please help me understand the logic flow of the follow pass thru variable provide by my billing provider? I am newbie to cgi

There are 10 user variables which you can pass-thru the signup system. Our signup system will not modify these variables, but will merely POST them back to your cgi. These variables are named:

user1, user2, user3, ... user10

as in the pass-in variables, you can send pass-thru variables to our system either via GET (appended to URL) or via POST. After a successful signup, these ten variables are posted back to your CGI.

user1 is a special case pass-thru variable. If it contains a value which starts with "http", it is considered to be a URL. Not only is it passed-thru as normal, but if the user successfully signs up, then on their confirmation page an invisible image is displayed with a source pointing to the URL in user1. This allows you to call a cgi on your end which has access to the normal CGI variables of the client (customer). If you needed to set a cookie on your customer's machine, or needed to determine his browser type, then you could do so by writing a simple script and passing its URL in user1.