Evaluates to True when the tempdb version store is greater than 15% of current TempDB size.
When snapshot isolation is in use, this can be an indicator that open transactions are preventing cleanup of the version store. The store is managed at the instance level, so an open transaction in one database will prevent cleanup for all databases.
To find open transactions, run DBCC OPENTRAN or query the sys.dm_tran_xxx dmvs.
See Also:
http://www.brentozar.com/archive/2013/01/implementing-snapshot-or-read-committed-snapshot-isolation-in-sql-server-a-guide/