This metric measures the number of sort warning events that occurred in the last hour. Sort warning operations that do not fit into memory can degrade query and server performance because multiple passes over the data are required to sort the data. It is important to optimize queries by removing them.
For more information, see http://www.simple-talk.com/sql/learn-sql-server/showplan-operator-of-the-week—sort/.
Note: This metric relies on SQL Server default trace. If this is not enabled, the metric will not work.