AREO* - Advisory Reorg Pending : How do I clear this flag ?

Have you ever seen this flag set while you displayed your database ?

How does this flag get set on a tablespace ?
If you are on DB2 8 for z/OS - You would have increased the length of a column from CHAR(05) to CHAR(10) for example
If you are on DB2 9 for z/OS or higher - You would have appended a column to a DB2 table or increased the length of a column from CHAR(05) to CHAR(10) for example

Is the data in the table still accessible while the flag is active ?
Yes, you can use the table for your application needs without any problem.

What are the different ways of clearing this flag ?

REORG:
It says Advisory Reorg pending, so a REORG will clear the flag for sure. You can perform a REORG with SHRLEVEL CHANGE, REFERENCE or NONE to clear this flag.

REPAIR:
REPAIR utility with its SYSIN DD control having REPAIR SET DATABASE.TABLSPACE NOAREOPENDSTAR will clear the flag

Unlike other force start methods which would clear a copy pending or check pending flags, Is it possible to clear the AREO* flag using similar force start methods ?
No, it is not possible. Well try to clear the flag using this method. The output would display :

DSNI015I  -PRDN OBJECT PRDNTS01 IS ALREADY STARTED FOR RW ACCESS. THE          
OBJECT WILL REMAIN STARTED FOR RW ACCESS                                       
DSN9022I  -PRDN DSNTDDIS 'START DATABASE' NORMAL COMPLETION         

The above display message states that database PRDNTS01 is started for RW access. Perform the display again on the same - you will see that AREO* status is still set on the tablespace, it won't vanish that easily.

Summary:
As a DBA, REORG should be the optimal solution for clearing the AREO* status, remember you get a free REBUILD of the index if you perform a REORG as compared to a REPAIR.