Well being an animator I knew what the IK handles were when I saw them and is an addition to the hierarchy that is not really need directly in Unreal 4 but was added by Epic to help make authoring of animations for their game like Fortnite and Pargagon easier and more efficient.
What might be confusing is the naming convention being used by calling them IK’s but what they are in reality is end effectors that are linked to the root of the rigging and constrained to limb end as a fixed target that is not interpolated by Unreal 4.
A good explanation.
The general behavior of UE4 is it interpolates everything but because how the IK is linked the left hand (end effector) will always be referenced to the right hand as it’s child and can be used as a target for a 2 bone IK to always keep the left hand bolted to say a weapon when doing something like an aim offset.
How they are linked up is of little relevance as to how they can be used once imported into Unreal 4 but rather connected depending on what application you are using to author the animations.
Why is the hierarchy
hand root
IK GUN
L Hand
R Hand
The ik_hand_root is not really the root but is linked to the root of the character. This allows the hierarchy to move relative to the character root when authoring root motion.
ik_hand_gun is linked to the ik_hand_root but is constrained to the right hand. Typical use would be arming and unarming as well as constrained to the right hand to say operate a bolt operated weapon.
ik_hand_l is linked to IK_hand_gun but can be constrained to a different joint. A typical use would be a weapon constrained to the ik_hand_weapon and ik_hand_l free to do a left hand reload.
ik_hand_r is linked to ik_hand_gun which in turn is constrained to the right hand. A typical use would be a right hand reload.
More or less though if you can not think of a reason as to why you need them then you don’t as they are there to make the animators life easier.