Aggregate Streams aggregate input data. In the input pipe a grouping is defined, and an output record is produced for each key value combination that is produced by the grouping.
Note that simply aggregations are better performed using aggregate pipes.
Aggregate Streams are functionally identical to merge streams, but by convention when there is only one input an Aggregate Stream is used to display as a 
on the model view - this often helps to clarify the purpose of the stream in the process.