Art - Caster is a special effect, played upon a unit that performs an ability. Therefore, you can attach that special effect to any of the attachments point, which are listed down there.
Art - Special is used by some abilities, such as ,,Carrion Swarm". In this case, the carrion swarm spell will create a special effect upon damaged units.
Art - Effect is the effect that will be displayed upon your hero before he/she casts a spell or starts to channel one. Applies for targets, points, no orders. This effect is used by Flame strike for an example.
The uses of so many attachment points are logical. Maybe you want to attach a blade to left or right hand, you certainly don't want it attached to head or something.