akSource and akProjectile can be None under various circumstances:

• This reference is an Actor:

  • akSource can be None if hit by a projectile attack where the projectile was not fired by a weapon or spell
  • akSource can be None if hit by a bash attack from a carried light object (e.g., a torch)
  • akProjectile can be None if hit by a melee attack.
    • In fact, at least in SE, akProjectile appears to always be None if the reference is an Actor, although it does work as expected for some other types, such as Activators.

• This reference is not an Actor:

  • akSource can be None if hit by a projectile attack where the projectile was not fired by a weapon (for example, a magic spell with a projectile component).
  • akProjectile can be None if hit by a melee attack.

Also, if this reference is an Actor and the projectile was caused by a weapon enchant, the enchanted weapon will be in akSource.

• This event is called multiple times when akSource has associated magic effects. If a sword has an enchantment with 2 effects, OnHit will be called 3 times - once for the physical damage of the sword and once for each magic effect. In all cases, akSource is the sword and not the enchantment. This also applies to spells (one hit for the spell projectile, and one for each associated magic effect).
• Keep in mind, that akAggressor data is somewhat restricted in this event, and not all functions can be run on it. For instance, you cannot call ObjectReference.GetAnimationVariableBool(...) and its variations on akAggressor.

• ObjectReference