Normal sudoku rules apply. The grey line in the central box is a thermometer. Digits along that thermometer must increase from the bulb to the tip, but the positions of the bulb and the tip are hidden. In the outer three rings, the sum of the digits along a ring between two 9s must always be the same throughout the puzzle. For the avoidance of doubt, those two 9s could be the same 9 if a ring has only a single 9 in it.