In the Character Creator, you are able to choose a type of shield when you purchase a shield from a class. For example, if I purchase a Tower shield, the Creator will ask me to select a type of shield (Coffin or War Door, for example). Mechanically there is no difference between types of shields within a class—the choice is purely cosmetic. You can ignore shield types if you do not use the Creator.
Shield classes have a Mighty requirement, which means your Mighty needs to be equal to or greater than the Mighty requirement in order to benefit from wielding the shield. If your Mighty is too low, then the shield affords no defense bonus, and you cannot use its tactic when parrying.
Magic Point (MP) Penalty
Wielding shields is disruptive to spellcasting. You can cast spells and carry a shield, but if you do so, the shield imparts a magic point (MP) penalty that increases the MP cost of any spell or maleficence you cast. The MP penalty reflects how exhausting it must be to think magically while sweating the weight of all that gear. For example, if your shield's MP penalty is 3, then any spell you cast costs 3 more MP. This penalty stacks with armor.