Custom channels is a very useful feature for Google AdSense publishers where they can track their AdSense performance by assigning ad units to channels based on their unique criteria such as colors, formats, or placements.

A sample implementation is to create a channel for each ad position, and see which one performs best. You can create channels such as example.com-top and example.com-bottom. You can also create channels such as color-red and color-blue and then use a custom script to rotate ads using the colors to see which one gets higher CTR.

To create a custom channel, go to My ads → Custom channels from your Google AdSense dashboard. From there click on the Create custom channel button.

In the Add new custom channel window, enter the name of your custom channel based on their criteria. The example below is to track the banner ad which is normally placed on top of a page. Click the 'Save button to create your channel.

Here you have the option to directly add ad units to the newly created channel, or you can add to it at later times.

When creating an ad unit, you can assign it to a channel in the Custom channels section. You can assign your AdSense ad unit to multiple custom channels.

To view the performance of your custom channels, go to Performance reports → Custom channels from your Google AdSense dashboard. There you can see performance report such as Page views, Clicks, Page CTR, CPC, Page RPM, and Estimated earnings for each channel, rather than just an overal view of your AdSense earning.