What means google_ad_channel?

google_ad_channel is an Adsense label that enables you to view detailed reporting about the performance of a particular advertising unit.

When creating an ad unit, you can attach a channel to it, and then you can start keeping track of that channel next time you log into Adsense. It can help you monitor your Adsense earnings between ad links unit or ad unit.

It is not a requirement, you can not use it if you don’t want. It is just for tracking purposes.