Many Sudoku variants involve clues which indicate which number is larger in two adjacent cells. In this puzzle for any two adjacent cells inside the same three-by-three cage, arrows point from cells containing larger numbers to those smaller numbers. The catch is that arrows appear if, and only if, the larger number is exactly one, two or three greater than the smaller one. Thus lack of an arrow between such cells reveals that the numbers are four or more apart. For example, here's one possibility for filling in a sample three by three cage:
For a full puzzle try:
If you're having trouble finding a place to start, there's a simpler version with some of the cells filled in at the start, and more information at my puzzle blog, Quadrata: https://quadratablog.blogspot.com/