Final Fantasy IV: The After Years enemy types

Final Fantasy IV has 8 different Enemy Types, which affect how certain pieces of equipment deal extra damage towards enemies of particular types, or how the enemies deal less damage. Specifically, damage is either multiplied by four, or quartered, respectively. In the case of the Undead enemy type, several other flags are added.

Machine
Against Machine enemies, Hammers deal extra damage.

Undead
The Undead type is the most complex of the Enemy types, as it covers more than just damage multipliers based on equipment. Undead enemies are also damaged by healing effects, such as Cure, and both Drain and Osmose have their effects reversed; their effects on Undead enemies damage HP and MP (respectively) and heal the Undead targets. Consequently, while Drain equipment technically has its damage multiplied by 4 against the Undead, this actually heals the opponent four times as much.