Dimensions sets make the report design process easier and more streamlined by limiting the number of dimensions in the Dimensions window.
Having a more manageable set of dimensions enables the user to design reports in a quicker fashion.

 

http://www.youtube.com/watch?v=CPV4W1tvEbk&list=PLBE027591B1C070D6&index=1&feature=plpp_video

 

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