Used for grouping records in the report. Each time the grouping expression changes the current group ends and the new one begins. Each grouping expression specifies condition for separate group level, i.e. Grouping Expression 1 specifies condition for group of the first level (or simply Group 1), Grouping Expression 2 specifies condition for group of the second level (or simply Group 2) etc.
You should use sorting to be sure that the grouping is correct.