The official rules are:
Each row, column, and block must contain each digit 1-9 exactly once. Since the middle block is missing, rows and columns that intersect the middle block will have only six visible numbers. In addition, each “Worm” in the puzzle contains numbers that increase from tail to eyes (although not necessarily consecutively). For example, a worm of length four could contain 2, 5, 7, 9 in that order, from tail to eyes.
Sudoku posted with permission. Thanks!