Thanks for the response.
this question i also posted in https://productforums.google.com/forum/#!msg/webmasters/GgJMWJc5qAs/-1TmOEC4-30J
And i got reply from Christopher Semturs - Google Employee
B) why for different content? Can you give a concrete example on where you consider this to be useful?
The point is he said that you can use on domain level.