RMAN automatically backs up the current server parameter file in certain cases. The BACKUP SPFILE command backs up the parameter file explicitly. The server parameter file that is backed up is the one currently in use by the instance. Make sure the target database is mounted or open.
The database must have been started with a server parameter file. If the instance is started with a client-side initialization parameter file, then RMAN issues an error if you execute BACKUP ... SPFILE.
The following example backs up the server parameter file to tape:
#Connect to RMAN
#Connect RMAN to target database
CONNECT TARGET SYS/pass@TEST1;
#Connect RMAN to recovery catalog (optional)
CONNECT CATALOG rman/cat@devdb;
#Issue RMAN BACKUP SPFILE command.
BACKUP DEVICE TYPE sbt SPFILE;