Item Id is a packed identifier which, in fact, stores more than a single value
It has a following structure:
Material ID is related to a quality of an item. Magic Item doesn't have this.
Kind ID is related to a kind of an item. It could be a Magic Item, Weapon, Shield, or, in all other cases, a Wieldable.
Rarity ID is related to a chance to find this kind of an item in the game, and, surprisingly, affects item's parameters and price similarly to a Material. Magic Item doesn't have this.
Item ID is, in fact, an identifier in one of the following enumerations -- Weapons, Shields, Wieldables -- in the case of Kind ID != Magic Item
Magic Item ID is, in fact, an identifier in Magic Items enumeration in the case of Kind ID == Magic Item