A DigitalPoint Forums thread discusses a new feature that Google AdSense added last night, named "Ad Placements." Ad placements are specific groups "of ad units, defined by you, on which an advertiser can place their ads."
You can see the new addition, if you log into Adsense and go to your channels page. At the top it has the following message:
NEW! We've improved custom channels - now, you have the option to enable any custom channel to be targeted by advertisers. You can also update any existing custom channels to allow advertiser targeting. Learn more about targetable custom channels.
It links to an AdSense help file: How can I make a custom channel into an ad placement? And there are also more details at What is an ad placement?
Google said that they are expecting "that creating ad placements will increase the visibility of your pages to advertisers who are creating site-targeted campaigns."
Thanks to Tom for also emailing me about this.
Forum discussion at DigitalPoint Forums.