I run a website with several different categories. I never thought about creating custom adsense channels for different branches of my taxonomy so that adsense advertisers can target specific niches. I do know many advertisers are running placements on my website.
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I'm curious how big of a revenue boost (or decline) you may have seen by adding custom channels? How granular did you go with your channels? It looks like I could easily do 18 channels, but one would be a "catch all".