To UNDO (rollback) a transaction means that the transaction is undone. The status of the database is reset to the consistent status it had before the transaction started.
For the UNDO, the original values of the data objects that the transaction was going to change, meaning the values from before the transaction was started, are stored for each transaction. This information is called an undo log entry or a before image entry.
Before a modifying SQL statement is carried out, the undo log entries are stored in an undo log file in the data area. Each transaction can access its undo log entries at any time.
liveCache database instances also use undo log entries to enable them to read data consistently without locking. When a transaction has finished, the liveCache database instance needs to keep all the undo log entries belonging to the transaction until all the other transactions that access the same objects have also finished. This is why, with liveCache database instances, the undo log entries are transferred to history management when the transaction that they belong to finishes.
See also:
Using Log Entries at Restart or Recovery