The rules of a center-dot Sudoku are very similar to the rules of X-Sudoku puzzles and Hyper
Sudoku puzzles. It is a standard Sudoku puzzle, but the center dot of each 3x3 region is filled in grey,
and they make their own region, i.e. they additional grey cells must contain the numbers 1-9 once
and only once.
This is an example of a finished center-dot Sudoku, and you can see that the grey center cells do
contain the numbers 1-9 once and only once.