You can create a custom channel for each piece of Adsense code you place on your site. It's used to know what areas are performing better or worse than others.
Okay I have three adsense ads, The top ad I named the channel - topad Middle ad- Midcontent Bottom ad - footerad at the end of the day I check my stats I would click on view top channels and I can see which ads were clicked more, which one gave more revenue, and from this information it allows me to tweak my ads for optimal performance.