History files are internal database files that contain references to the undo log files of completed transactions. The history files show which undo log file belongs to which completed transaction.
The initial history files are generated in the data area when the database instance is created. At each savepoint, the updated history files are backed up in the data area.
For performance reasons, there are always multiple history files open in parallel. The number of history files is calculated by the database system based on the general database parameter MAXUSERTASKS.
See also:
Concepts of the Database System