Basicly, 3D animation relies on the animator to 'assign' what areas of the model are assigned to what 'bone' so, obviously you want the 3D faces for the head to be different from the 3D faces for the neck when you're animating.
Essentially you 'paint' what areas you want to be manipulated by each bone (fading the colours into one another for a smooth realistic body-type frame).
As shown below, the "painted" areas are where the skeleton will know to manipulate the 3D model.
Hair (for the most part in games) follows this rule also, where certain segments of the hair will be weighted to the head, neck, back or any other joints that are assigned for the animation skeleton.