Some good question and answers that i got from different oracle forums 

1.I am not able to get much details about the use of STANDBY_ARCHIVE_DEST . Can anyone clarify the same ? Is it the place where archived logs of standby redo logs are stored ? In that case if we have both STANDBY_ARCHIVE_DEST and LOG_ARCHIVE_DEST set then whether both of them are used , if so in which cases each destination is used ?

Ans:- If both STANDBY_ARCHIVE_DEST and LOG_ARCHIVE_DEST_n are specified, then STANDBY_ARCHIVE_DEST will be used, however it is sufficient enough to simply use LOG_ARCHIVE_DEST_n.  STANDBY_ARCHIVE_DEST is actually deprecated in 11g.

2. what is the need of standby redo log file??
Ans:- Standby redo logs are necessary for the higher protection levels such as 
Guaranteed, Instant, and Rapid. In these protection modes LGWR from the 
Primary host writes transactions directly to the standby redo logs. 
This enables no data loss solutions and reduces the amount of data loss 
in the event of failure. Standby redo logs are not necessary if you are using 
the delayed protection mode. 

If you configure standby redo on the standby then you should also configure 
standby redo logs on the primary database. Even though the standby redo logs
are not used when the database is running in the primary role, configuring 
the standby redo logs on the primary database is recommended in preparation 
for an eventual switchover operation. 
Standby redo logs must be archived before the data can be applied to the 
standby database. The standby archival operation occurs automatically, even if 
the standby database is not in ARCHIVELOG mode. However, the archiver process 
must be started on the standby database. Note that the use of the archiver 
process (ARCn) is a requirement for selection of a standby redo log.

You must have the same number of standby redo logs on the standby as you have 
online redo logs on production. They must also be exactly the same size.

The following syntax is used to create standby redo logs:

SQL> alter database add standby logfile
2 '/export/home/oracle/temp/oracle/data/srl_1.dbf' size 20m;

Database altered.
SQL> alter database add standby logfile
2 '/export/home/oracle/temp/oracle/data/srl_2.dbf' size 20m;

Database altered.

SQL> alter database add standby logfile
2 '/export/home/oracle/temp/oracle/data/srl_3.dbf' size 20m;

Database altered.


 3. what is the importance of DBID in rman??

In the case that you need to restore control file or spfile using RMAN , you need to set the DBID first.

4. RMAN Not able to find Controlfile autobackup. what may be the reason?
If Your source database has the controlfile autobackup set to a non-default location , for example
CONFIGURE CONTROLFILE AUTOBACKUP ON;
CONFIGURE CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO '/backup/%d/controlfile_backup_%d_%F';
Set the controlfile autobackup format on the target and try to restore it.

Refer this http://waseemslearning.blogspot.com/2011/03/rman-06563-control-file-or-spfile-must.html

Suppose on the target server, you have copied the controlfile autobackup to location "/u01/autobackup/", then on the target server do as:
RMAN>run
{
set controlfile autobackup format for device type disk to '/u01/autobackup/%F';
restore controlfile from autobackup;
}