The reason we have a reserved loot list is because a select few items have a significant effect on the guilds ability to raid on how effective we are if the items go first to certain classes. Reserved list items can either be specific items or a class of items. If you won a shield or any other item that only goes in one slot in the current expansion via a reserved bid win, you can not enter another reserved bid for the same slot of the same type.
Bards:
Orb of Tishan
http://everquest.allakhazam.com/db/item.html?item=84
Rogues:
Piercing weapons
Rangers:
Bow's
Warriors:
Shields