For most standard reports of the system, developers have implemented the ability to set fields by which data in the report will be grouped. Predefined groupings are available to the user in the Quick Settings panel:
In defaultGroup by section user may select attributes which would be used for groupping data in report and aslo user may set order of grouppings (move them using the up and down arrows 
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Grouping of data in the report by selected fields will be applied in the order in which they are located in the Group by section.
In case when user need to add additional grouppings in report - he may use Change report Variant option (menu More Actions - Change Report variant):
For making setting about groupping it is needed to open Groupping tab