backing up as copy of image copies using from tag

RMAN>  backup as copy
2> datafile 2
3> from tag 'bkp_copy_df2'
4> tag 'copy_bkp_copy_df2';

Starting backup at 17-may-2014 11:22:37
using channel ORA_DISK_1
using channel ORA_DISK_2
using channel ORA_DISK_3
using channel ORA_DISK_4
using channel ORA_DISK_5
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of backup command at 05/17/2014 11:22:37
RMAN-06577: FROM TAG option may only be used with datafile copies

Solution:-
We need to include copy of datafile clause instead of datafile only

RMAN> backup as copy
2> copy of datafile 2
3> from tag='bkp_copy_df2'
4> tag='copy_bkp_copy_df2';

Starting backup at 17-may-2014 11:31:52
using channel ORA_DISK_1
using channel ORA_DISK_2
using channel ORA_DISK_3
using channel ORA_DISK_4
using channel ORA_DISK_5
channel ORA_DISK_1: starting datafile copy
input is copy of datafile 00002: +DATA/cvrman/datafile/sysaux.312.847797559