Contextual Publisher pop under traffic works on completely different fundamentals and technology approach, here URL targeting options are not available so targeting is only done via Keywords. Adserving technology identifies the keywords appearing in Meta tags to serve relevant popunder ads. So if you looking to advertise a site for Europe shopping keyword, ads will appear if site has Europe shopping keyword mentioned in his meta tags. Such precise targeting allow you to reach targeted audience and hit better conversion rate with quality targeted traffic.