Normal sudoku rules apply. Adjacent digits on a line must form a valid chess move by the indicated piece on a 3x3 numpad. For example, a 1 on a knight line may only have the numbers 6 and 8 next to it so 6-1-8-1 would be valid. Pawns can only move exactly one cell up or one cell down, so 2-5-8-5 would be a valid pawn line.