It's a new features of Oracle 9i.
alter session workarea_size_policy = manual; alter session workarea_size_policy = auto;
If WORK_AREA_SIZE_POLICY = AUTO, the hash_area_size and sort_area_size setting are ignored, Oracle will set them automatically, using 10MB when appropriate, 1MB when that is better and 100KB when that make sense. In fact, the amount used can vary from statement to statement in your session as the load goes up and down over time.
This parameter is extremely useful for huge sorting/grouping systems. The parameter informs the server that it should take over PGA memory management.