A threshold on the depth in bytes of the queue being measured at
which a trigger is generated to the dropping algorithm, unless
diffServAlgDropType is alwaysDrop(5) where this object is
ignored.
For the tailDrop(2) or headDrop(3) algorithms, this represents
the depth of the queue, pointed to by diffServAlgDropQMeasure, at
which the drop action will take place. Other algorithms will need
to define their own semantics for this threshold.