From 11g onwards , standby database can be opened in read write , application can test and then it can be converted back to physical Standby. Changes made will be lost. Standby is not required to have flashback database.
Convert to Snapshot Standby
ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;
SELECT flashback_on FROM v$database; ==> NO
ALTER DATABASE CONVERT TO SNAPSHOT STANDBY;
ALTER DATABASE OPEN;
SELECT flashback_on FROM v$database; ==> RESTORE POINT ONLY
Back to Physical Standby
SHUTDOWN IMMEDIATE;
STARTUP MOUNT;
ALTER DATABASE CONVERT TO PHYSICAL STANDBY;
SHUTDOWN IMMEDIATE;
STARTUP NOMOUNT;
ALTER DATABASE MOUNT STANDBY DATABASE;
ALTER DATABASE RECOVER MANAGED STANDBY DATABASE DISCONNECT;
SELECT flashback_on FROM v$database;