How are ad placements different from custom channels?
Think of ad placements as ‘enhanced’ custom channels. To create an ad placement, you create and implement a custom channel just as you normally would. The only change is the additional step of providing attributes and an accurate description for your ad placement. On the ‘Channels’ page in your account, click the appropriate ‘edit settings’ link on the right. As you can see below, there will be a box labeled ‘Targeting’ on the next page which, once checked, will show this channel to advertisers as an ad placement. From there, you’ll just need to provide some details on where the ad unit appears on your site:
Advertisers will see the ‘External name’ and ‘Description’ along with the domain on which that placement appears, so a meaningful name and detailed description clarifying where the ads will show can make your placement more appealing. Be sure to highlight your most valuable and best-performing ad units as placements. For example, try a description like ‘Ads will appear in a 300×250 rectangle embedded within articles on the Travel section’ or ‘Ads will appear only in the leaderboard ad unit directly above any page on the site.’
Read the Complete Post on the Adsense Blog.
