�         CI Enqueue - The CI enqueue is the Cross Instance enqueue (a.k.a. enq: US - contention) and happens when a session executes a cross instance call such as a query over a database link.
 

�         FB enqueue � This is the Format Block enqueue, used only when data blocks are using ASSM (Automatic Segment Space Management or bitmapped freelists).  As we might expect, common FB enqueue relate to buffer busy conditions, especially since ASSM tends to cause performance problems under heavily DML loads.
 

�         HV enqueue � The HV enqueue (a.k.a. enq: HV - contention) is similar to the HW enqueue but for parallel direct path INSERTs. 
 

�         HW enqueue � The HW High Water enqueue (a.k.a. enq: HW � contention) occurs when competing processing are inserting into the same table and are trying to increase the high water mark of a table simultaneously. The HW enqueue can sometimes be removed by adding freelists or moving the segment to ASSM.
 

�         KO enqueue � The KO enqueue (a.k.a. enq: KO - fast object checkpoint) is seem in Oracle STAR transformations and high enqueue waits can indicate a sub-optimal DBWR background process.
 

�         PE enqueue � The PE enqueue (a.k.a. enq: PE � contention) is the Parameter Enqueue, which happens after �alter system� or �alter session� statements.
 

�         PS enqueue � The PS enqueue is the Parallel Slave synchronization enqueue (a.k.a enq: PS � contention), which is only seen with Oracle parallel query.  The PS enqueue happens when pre-processing problems occur when allocating the factotum (slave) processes for OPQ.
 

�         RO Enqueue � The RO enqueue is the Reuse Object enqueue and is a cross-instance enqueue related to truncate table anddrop table DDL operations.
 

�         SQ enqueue � The SQ enqueue is the Sequence Cache enqueue (a.k.a. enq: SQ � contention) is used to serialize access to Oracle sequences. 
 

�         SS enqueue � The SS enqueue is the Sort Segment enqueue (a.k.a. enq:SS � contention) and these are related to the sorting of large result sets.
 

�         SS enqueue � The SS enqueues are Sort Segment enqueues (a.k.a. enq: SS � contention), and occur when a process is performing a large sort operation. 
 

�         ST enqueue � The ST enqueue can be seen in a partitioned environment when a large number of partitions are created simultaneously.
 

�         TC enqueue � The TC enqueue is related to the DBWR background process and occur when �alter tablespace� commands are issued.  You will also see the TC enqueue when doing parallel full-table scans where rows are accessed directly, without being loaded into the data buffer cache.
 

�         TM enqueue � The TM enqueue related to Transaction Management (a.k.a. enq: TM - contention) and can be seen when tables are explicitly locked with reorganization activities that require locking of a table.
 

�         TQ enqueue � The TQ enqueue is the Queue Table enqueue (a.k.a. enq: TQ - DDL contention) and happens during Data ump (export import) operations.
 

�         TS enqueue � The TS enqueue is the temporary segment enqueue (a.k.a. enq: TS � contention) and these enqueues happen during disk sort operations.  
 

�         TT enqueue � The TT enqueue (a.k.a. enq: TT � contention) is used to avoid deadlocks in parallel tablespace operations.  The TT enqueue can be seen with parallel create tablespace and parallel point in time recovery (PITR)
 

�         TX runqueue � The TX enqueue is the transaction enqueue (a.k.a. enq: TX � contention) and is commonly related to buffer busy waits, in conditions where multiple transaction attempt to update the same data blocks.