The bench dip is a suitable substitute for those who do not have access to a dipping station at their gym, but is also a great exercise in its own right. As with the traditional dipping exercises, how we position ourselves during the dip influences which muscle groups we primary target.
With that said, the bench dip is likely to tax the triceps and front (anterior) shoulder head greatly, regardless of if we take a narrow or wide grip on the bench. This is because the torso is forced to remain upright during the bench dip, with the athlete unable to lean forward enough to place more emphasis on the chest.
Despite being heavily taxing on the triceps and shoulders, we can alter the emphasis somewhat by altering the width of the grip we take on the edge of the bench. A narrow grip will reduce chest involvement, bringing the vast majority of the stimulation to the triceps. On the other hand, by taking a wide grip on the bench we can incorporate the chest, specifically the sternal head, to a greater degree.