Think of a stream as a series of placements a student is assigned to within a placement group. Streams make it easy to see if a student is assigned placements in the Areas they need, for the duration required and in the sequence you want.
With one "allocate" click one or more students are allocated to all of the placements in their stream patterns. Your Sonia business rules are enforced, making it easy to spot students who couldn't be allocated to some or all of the placements you planned for them.
The following three videos (around 10 minutes each) walk through creating streams and allocating students.