The document describes the Oracle undo segment and how it tracks changes to data in transactions.
1) It shows the initial state when a value of "A" is entered into a table column.
2) It then shows an update transaction that changes the value from "A" to "B", with the undo segment recording the before image of "A".
3) A second update transaction is shown, changing the value from "B" to "C", with the undo segment recording the before images of "B" and "A".