making a custom channel into an ad placement channel

You can make a custom channel into an ad placement channel in your site. That channel would be targetable by advertisers. They will see the ad placement you defined among the available places on which to place their ads.

First you Log into your account at http://www.google.com/adsense.
Click on the AdSense Setup tab, then click on the Channels page.
Choose the appropriate product sub-tab and make sure you've selected custom channels.
Click the radio button next to the channel you'd like to make targetable, then click the corresponding 'edit settings' link.
Click the checkbox marked Show this channel to advertisers as an ad placement. enter a little but accurate and informative description for your channel.
Click on Save channel.