A few weapons are fairly random in their accuracy and require you to roll a Games Workshop Scatter dice to determine where they land. This Scatter dice is marked on four sides with an arrow, and on two sides with a special 'HIT' symbol. Simply roll the Scatter dice near the target point, if an arrow is rolled this shows which direction the shot has deviated in. If a Hit symbol is rolled this generally means the shot is bang on target. Some weapons may scatter automatically, with no chance of a direct Hit, in which case you will find a small arrow on the Hit side to determine the direction.
Scatter Dice
Source: 3rd-A-Palooza (3.5) Rulebook v1.4, p. 1