I've used Adsense for years and I've never managed to find out how to do this, which definitely should be available.
Imagine you have 10 articles on your site which = 10 URL channels because you wish to track how each article performs.
You have URL Channel 1, 2, 3... all the way to 10.
You have 3 Ad Units: Top, Middle and Bottom.
How do you find out how well the top Ad Unit is performing on say, URL Channel 4, 7 and 9?
The Top Ad Unit might not work well on all articles and therefore you can remove it on those and leave it on the rest.
The only thoughts I've had is to create an obscene amount of Ad Units... like 3 for every single article. But that is just daft.