What does Copy AdSense targeting do?

If you enable your placements for AdWords targeting you will have the option to enter information describing the placement (viewable to AdWords advertisers), or copy pre-exisiting information from an AdSense channel in your AdSense account.

By selecting Copy Adsense targeting and selecting a channel, you can copy AdSense channel information to your placement and continue to generate revenue from AdWords campaigns that are targeting your site.

Copying AdSense targeting settings will not affect your channels in your AdSense account. However, please note that any channels that you copy will appear twice in your AdSense reports.