Yet another question about AdSense...

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So the AdSense does not allow links on your site that are linked to PTC/PTR etc etc sites, but yet I have seen many blogs where people write about those sites, usually the site name is linked to their referral link on this site and they also have AdSense adverts on their sites. How is that possible? Does the AdSense only forbid you of posting direct referral links in text or how does it work?