Spell level absorption is an effect that absorbs spells before they can affect the creature with this protection. When a spell is absorbed, it simply does not affect the targeted creature, but it could still affect other creatures. Absorption has a limit to how many total spell levels can be absorbed before the effect is depleted, and there may also be a maximum level of an individual spell that can be absorbed. (If a spell's level meets the individual maximum but exceeds the remaining total levels that can be absorbed, the spell will be absorbed, depleting the effect.) Common sources of this effect include the spell mantle line of spells.
This is part of the spell resistance check, so only absorbs spells that perform that check. In addition, the total spell levels that can be absorbed is reduced for each spell resistance check, even if multiple checks are made in the same spell. (So, for example, flame arrow reduces that limit by 3 for each arrow , and if an arrow causes the spell level absorption effect to be depleted, the remaining arrows can affect the target.)
There is also an option for this effect to limit its protection to spells of a specific school, but this is not used by any standard spells.
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This type of effect is created with the command EffectSpellLevelAbsorption() when the second parameter (total spell levels absorbed) is greater than the default value of zero. (Leaving the second parameter as zero gives a spell immunity effect instead.)