The standard rules of Sudoku apply to an Entropy Sudoku puzzle.

Also, the low values (1, 2, 3), the medium values (4, 5, 6) and high values (7, 8, 9) are categorised as 3 different groups. Every 2×2 box has to contain at least one value from the low, medium and high groups.

This is an example of a completed Entropy Sudoku, you can see that every 2×2 box does contain at least one value from each group.

This grid is partially filled - and 4 of the cells are highlighted because there is a mistake with those 4 cells.

You can see that every other filled group of 2×2 cells contains a value from each of the low, medium and high groups.

The highlighted cells are missing a value from the high group, i.e. it does not contain a 7, 8, or 9.