Elemental Seal is a Job ability available to Black Mages who reach the 15th level or greater in Final Fantasy XI. It can only be used once every 10 minutes. When activated it will increase the potency of the next Black Magic spell cast. If this is a damage spell this could potentially increase the quantity of damage significantly. This is true because the elemental seal increases the total damage potential and also makes the spell less resistible.
Because of this, the Elemental Seal is often used just before casting a spell that does no damage, such as Sleep spells. Since it makes the spell less resistible, the enemy creature is less likely to have a full or partial resist (which would lessen the duration of the sleep). Since sleep spells create a lot of enmity that is undesirable and very potentially deadly for mages, it is best to try to inflict the sleep status on the creature on the first try.
Once a spell is cast, the Elemental Seal is expended and the status effect disappears from the player's screen. The recast time is based on the time the Elemental Seal was called for, not when it was expended. The recast can be reduced with merit points.