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yesterday i was configuring a couple of new placement ad groups and was looking for space for an energy drink. i came across tons of body building sites - and i wished they all had a segment called supplements or dietary aids so i could really use a high bid.
i think webmasters tend to think of placement, and less so about marketability of certain subpopulations of their visitors.
they demonstrate both Custom Channels and URL Channels in the above help file. imo, the URL channels are a great way to increase your adsense income by permitting better targeting on the other end for your space. the better appropriate sections can be targetted, the more valuable your space is too advertisers and google, and therefore to you too.