While some doors may be ok with just a bore, many times hollow metal doors will utilize a door closer or panic hardware. In order to use these, additional reinforcements for this hardware is needed to ensure the door will stand up to the continual usage. There are two types of reinforcement possible.
With a bore, often times a door closer will be used. Reinforcement will be added to the top of the door to account for the door closer. If panic hardware will be used, the door will be reinforced in the middle for the push bar as well as at the top for the door closer. To reinforce the door, additional material is welded to the back of the door skin for extra protection.