Normal sudoku rules apply. A digit in a circle indicates how many circles contain that digit. On an “index line”, the digit in the Nth cell along the line (starting from the diamond) indicates the position along the line where the digit N appears. Eg: 3214 would be a valid line; the 1st digit, 3, indicates that the 3rd cell contains a 1, the 2nd cell a 2, and so on.