The formatting of this information varies by platform.

Syntax:

%c The copy number of the backup piece within a set of duplexed
backup pieces. If you did not duplex a backup, then this variable
is 1 for backup sets and 0 for proxy copies.
If one of these commands is enabled, then the variable shows the
copy number. The maximum value for %c is 256.

%d The name of the database.

%D The current day of the month (in format DD)

%F Combination of DBID, day, month, year, and sequence into a unique
and repeatable generated name.

%M The month (format MM)

%n The name of the database, padded on the right with x characters
to a total length of eight characters. (AKA: Porn star alias name)
For example, if the scott is the database name, %n= scottxxx.

%p The piece number within the backup set. This value starts at 1
for each backup set and is incremented by 1 as each backup piece
is created. Note: If you specify PROXY, then the %p variable must
be included in the FORMAT string either explicitly or implicitly within %U.

%s The backup set number. This number is a counter in the control file that
is incremented for each backup set. The counter value starts at 1 and is
unique for the lifetime of the control file. If you restore a backup
control file, then duplicate values can result.
Also, CREATE CONTROLFILE initializes the counter back to 1.

%t The backup set time stamp, which is a 4-byte value derived as the
number of seconds elapsed since a fixed reference time.
The combination of %s and %t can be used to form a unique name for
the backup set.

%T The year, month, and day (YYYYMMDD)

%u An 8-character name constituted by compressed representations of
the backup set number and the time the backup set was created.

%U A convenient shorthand for %u_%p_%c that guarantees uniqueness in
generated backup filenames.
If you do not specify a format, RMAN uses %U by default.

%Y The year (YYYY)

%% Specifies the '%' character. e.g. %%Y translates to %Y.