History files are internal database files that contain references to the undo log files of completed transactions. The history files tell you which undo log file belongs to which completed transaction.
At each savepoint, the database system backs up the updated history files to the data area.
For performance reasons, there are always several history files open in parallel. The database system calculates the number of history files based on the general database parameter MAXUSERTASKS.
Concepts of the Database System