Google Explains Allowed Sites Option

Over the past couple of weeks Google Adsense has been rolling out a new option to users which allows you to specify which sites/ domains are allowed to publish ads using your code.

It wasn't very clear initially what was intended by this, but the Adsense Blog has provided some level of clarification

It's still not 100% clear if publishers will run into issues if they fail to make use of the new option.