many Adsense publishers may already know that an effective way to increase AdSense revenue is to place multiple ad units on content rich web pages. At the same time, there is a way to maximize the amount of revenue you use for multiple ad units, which is to ensure that the highest click rate of the ad unit is the first unit in your web page HTML code. May be a lot of publishers do not understand why this can increase revenue, the following explains how the Adsense system in the page configuration advertising.
at present, the front page of the advertising unit is always the first to show the highest advertising. At the same time, if you don’t have enough ads on your site, the first ad unit will give priority to advertising. This is why we have been suggesting that you use custom channels to track the effectiveness of each ad unit, to find the highest rate of the ad unit, and in the HTML code to put the ad unit in the front.
what we call "the first ad unit" is the first advertising code in the HTML code of the web page. Note that the first ad unit in the source code is not always the first ad unit that users see in the browser. For example, if you use the DIV tag in the code to set up, our system is also possible to the bottom of the page advertising unit as the first advertising unit.
source: Google Adsense Chinese blog