Dear stefano,
Thank you so much for your explanation!
I understand now. And I found a good picture that demonstrate this particular issue.
here is the image link for whom confused with the same problem.
http://image.sciencenet.cn/album/201404/23/151523b496kcef1enlzaln.jpg
http://image.sciencenet.cn/album/201404/23/151523b496kcef1enlzaln.jpg
best regards
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