%n : Specifies the database name padded with the right with 8 characters – ORCL3XXXX
%s : specifies the backupset. This number is a counter in controlfile that is incremented with each backupset.
It is started with 1 and unique for lifetime. Maximum 7 character long (9,999,999)
%p : specifies the backup set timestamp
%c : specifies the backup copy number.
%u : specifies an 8-character name constituted by compressed representations of the backup set number and
the time the backup set was created.
%D : specifies the current day of the month from the Gregorian Calendar in DD format.
%F : specifies the DBID, yerar, day, month and sequence be combined so that generated name is unique and
%M : specifies the current month from the Gregorian Calendar in MM format.
%T : stands for year, month and day in the format YYMMDD. This will always be 6 characters in length.
%U : specifies that guarantees uniqueness in generated backup filenames.
%Y : specifies the current year from the Gregorian Calendar in YYYY format.
Note: If you do not specify a value for the FORMAT parameter, then RMAN either creates the backup in the
flash recovery area if it is enabled, or in a platform-specific directory (for example, ?/dbs on UNIX) if a flash recovery area is not enabled. In either case, RMAN uses the variable %U to name the backup.
For more details follow the link: http://docs.oracle.com/cd/B28359_01/backup.111/b28273/rcmsubcl010.htm