How to create Adsense ad units correctly?

Question

contextual advertising

Actually questions on the topic:
- what will happen if one ad unit (of the desired format) is placed on several sites? What can affect what? Will efficiency drop?
- will it be more competent to create ad blocks for each site? How to group them?
- how to use client channels? why are they?
Share your experience, how does one work with Adsense? How do you optimize blocks (format, layout)?
Thank you.