In the original Famicon and NES releases, spells and attacks can have eight elemental properties: Fire, Ice, Lightning, Quake, Poison & Stone, Time, Death and Status. Since Final Fantasy Origins, Poison and Stone was split into two separate elements, and Status was split into six: Darkness, Paralysis, Confusion, Mind, Sleep and Silence.
A weapon can have multiple elemental properties, and a piece of armor can grant resistance to multiple elemental properties. However, spells and enemy abilities never have more than one property (spells like NulAll grant the targets resistance to specific properties, but the spells don't have any elemental properties themselves).
Weaknesses and resistances to elemental properties have different effects for damaging spells, status spells and weapons:
Spells and enemy abilities have a dramatically reduced hit rate against targets resistant to their properties, and have an increased hit rate against targets weak against their properties.
For status-inflicting spells, such as Quake, Stop, Break and Sleep, the hit calculation determines whether the spell succeeds or fails in inflicting status ailments.
Damaging spells deal half damage if they miss (for example, Fire deals 20-40 damage if it hits or 10-20 damage if it misses).
In addition to the hit modifiers, damaging spells and enemy abilities deal 1/2 damage against resistant targets, and 1.5x damage against weak enemies.
Status spells, such as Quake, Stop, Break and Sleep, have a dramatically reduced success rate against targets resist to their properties, and have an increased success rate against targets weak against their properties.
Attacking with a weapon which matches one of a target's weaknesses (such as attacking a Sahagin with the Coral Sword) provides bonuses in the attack formula. In the original NES/Famicom release, this is bugged, but it was fixed for the Origins release. Elemental weapons are believed to ignore enemy resistances.
Attempt to inflict Instant Death against an enemy. Always succeeds if the target has 300 or less HP and isn't resistant against the Death property. Always fails if the target has over 300 HP, or is resistant against the instant death property.
Status-based elements are different than status effects. Resistance to status-based elements does not guarantee any additional resistance to the status effect. If the effect is applied via a means that does not have the status element, a resistance does not eliminate or reduce the chance that the effect is applied. Examples of this are the non-elemental spells Sleepra and Slowra.
Attempt to inflict Blindness against an enemy. Always succeeds if the target has 300 or less HP and isn't resistant against the miscellaneous status property. Always fails if the target has over 300 HP, or is resistant against the miscellaneous status property.
Attempt to inflict Paralysis against an enemy. Always succeeds if the target has 300 or less HP and isn't resistant against the miscellaneous status property. Always fails if the target has over 300 HP, or is resistant against the miscellaneous status property.
In the original Famicon/NES versions, Dia does not function as an element. Instead, the spell is scripted to check whether the target has the Undeadtype. If so, it deals non-elemental damage to the target, otherwise, it has no effect.
Dia is, however, listed as an elemental weakness in the Bestiary in some remakes, including Dawn of Souls.
In the original Famicon and NES releases, XFER/Dispel was supposed to remove all elemental and status resistances, however it was bugged and non-functional. In the remakes, it was changed to instead remove all positive status effects.