When you advertise your Coop link you will earn credits. When you assign the credits you have earned to your website, it will display in other people's Coop links on all the traffic sources. So even if you don't advertise directly in a traffic source, another member here definitely does and that means your ads will be displayed there too.
Your Coop link is the link you can advertise in traffic exchanges and mailers to earn credits for your websites. It always displays another Harvest Traffic Coop member's ad, since that is how ads are distributed in all traffic exchanges. Other members will display your websites when they promote their Coop link. The Coop link displays the Harvest Traffic Coop top frame, and a website inside. The top frame has your referral ID attached to it.