Recovery Area Cannot Be Used For Archive Log?

I`m using the the version 11.0.2.3 for testing.  I read a line as below from the OCP book.

If none of the LOG_ARCHIVE_DEST_n parameters have been set, then transitioning. the database to archivelog mode will internally set LOG_ARCHIVE_DEST_10 to the flash recovery area, if it has been configured. In my database the fast recovery already been set as below.

SQL> show parameter db_recovery
 
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest                string      +FASTRECOVERY
db_recovery_file_dest_size           big integer 4248M

So I thought if I turn the database into archive log mode, then the log_archive_dest_10 will be set as the same value as db_recovery_dest.

SQL> alter database archivelog;
 
Database altered.
 
SQL> alter database open;
 
Database altered.
 
SQL> show parameter log_archive_dest
 
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
log_archive_dest                     string
log_archive_dest_1                   string
log_archive_dest_10                  string

See, after the database become archive, the log_archive_dest_10 still be empty.  So is there any thing wrong with my understanding?