Stream Aggregate
The Stream Aggregate physical operator optionally groups by a set of columns and calculates one or more aggregate expressions returned by the query and/or referenced elsewhere within the query. This operator requires that input is ordered by the columns within its groups.
If the Stream Aggregate operator groups by columns, a GROUP BY:() predicate and the list of columns appear in the Argument column. If the Stream Aggregate operator computes any aggregate expressions, a list of them will appear in the Defined Values column of the output from the SHOWPLAN_ALL statement or the Argument column of the graphical execution plan.