Suppose I have 2 sites - A & B. A is on greetings card and B is on share market. So the ads they will draw will definitely be different. Now my Q is, if I use the same ad code in these 2 sites, then will adsense be unable to deliver best ads to provide optimal earning ? Is it required to create different channels for greetings card and share market and add 2 different ad codes to them for better earning ?
AdSense is too smart. So, you don't have to worry about that. However, for your own benefit and to make things easier for you to track your perfomance, create another channel.
He means "URL Channel" (I think that's what they call it). The thing where you specify the URL you want to track, so AdSense can separate where your clicks are coming from in the reports. It doesn't affect the ad blocks or anything else - it's just like a filter.