If you want to track a specific channel (not just URL specific) are you required to enter both the URL channel and a specific custom channel ID, or just the latter?
No problem. Basically the URL channel is a "catch all" type of channel. It will show all the revenue, clicks etc. generated from that specific page. With a custom channel you can assign it to specific ad units (ad spots). So if you have a URL channel for "thispage.html" and you have 2 ad units on the page with a custom channel for each then the URL channel will show the earnings for both those ad units. Just bear in mind that if you have overlapping channels the totals might not be the same as your Total earnings for the day. So you'll most likely have to disable the URL channel(s) for pages with ad units with custom channels assigned, to get an accurate figure.