In Killer Sudoku there’s a new concept of a “cage” – a group of cells within a dotted line that make up a sum shown as a small number above. Notes/pencil marks are small numbers that you can enter into cells when you aren't yet sure of the final answer. You can update all of the highlighted cells together by using the keyboard or the touchpad as normal.

The Times Ultimate Killer Su Doku by The Times Mind Games

In other words, it (a) undoes all changes to cage combinations since the last cell change, (b) undoes the last cell change, and (c) undoes all changes to cage combinations since the previous cell change.This way, by applying the “45 rule” we filled two numbers on the empty grid and are off to a good start!

Daily Killer Sudoku

When the highlighted cells all belong to the same cage, the Combinations control shows its cage combinations.If fill uses cage combinations is enabled, the FILL button will only add the numbers 1 2 3 4 to cells within the cage. Make sure the sum of numbers in each cage is equal to the number in the upper left corner of the cage. If cage combinations are included, the Undo button undoes the last cell change and all changes to cage combinations since the previous cell change.