When you use an external backup tool to back up or unload data, then each backup or data stream gets an external backup ID. If a backup was created using a group of parallel backup media, every part of this backup has its own external backup ID.
The Database Manager writes the external backup ID to the log files of the Database Manager, dbm.knl and dbm.ebf. This ID is also displayed in the backup history.
The Loader writes the external backup ID to the Loader log file and to the system tables relevant to the Loader.
The external backup ID is needed when restoring or loading data, to request the required backup or data stream.