Operations Behind BACKUP ... PLUS ARCHIVELOG Command.

Backing Up Logs with BACKUP ... PLUS ARCHIVELOG
You can add archived redo logs to a backup of other files by using the BACKUP ...PLUS ARCHIVELOG clause. Adding BACKUP ... PLUS ARCHIVELOG causes RMAN to do the following:
1. Runs the ALTER SYSTEM ARCHIVE LOG CURRENT command.
2. Runs BACKUP ARCHIVELOG ALL. Note that if backup optimization is enabled, then RMAN skips logs that it has already backed up to the specified device.
3. Backs up the rest of the files specified in BACKUP command.
4. Runs the ALTER SYSTEM ARCHIVE LOG CURRENT command.
5. Backs up any remaining archived logs generated during the backup.

This guarantees that datafile backups taken during the command are recoverable to a consistent state.
To back up archived redo logs with BACKUP ... PLUS ARCHIVELOG:
After starting RMAN, run the BACKUP ... PLUS ARCHIVELOG command at the RMAN prompt . This example backs up the database and all archived logs:
BACKUP DEVICE TYPE sbt DATABASE PLUS ARCHIVELOG;
note: If backup optimization is enabled, then RMAN skips backups of archived logs that have already been backed up to the specified device.