What is the {$extra_data1} and {$extra_data2} for in a banner's Destination URL ?

The {$extra_data1} and {$extra_data2} can be used to forward some value to the merchant's website. 


What if a merchant has a site  www.merchantsite.com/?productid=12345  where he wants the product id to be changed dynamically?

In that case, he can set in a banner's Destination URL the following:
and then for click can be used:

and that will redirect to www.merchantsite.com/?productid=12345

The value by which the {$extra_data1} is supposed to be replaced has to be entered by the affiliate when he/she gets the banner code in the affiliate panel. So the merchant has to tell the affiliate the available values for that variable.