How does BoneSpace work in the Two Bone IK Skeletal Control?

I’m a bit confused how the End Effector setup works in the Two Bone IK control.

I can specify world space for example, but then there’s no way to tell the end bone what rotation it should be in, unless I set up a second controller separately.

Then there’s the Bone Space option which seems more desireable since there seem to be options for rotation. What bone does it connect to? I can’t think of why a bone would connect to another bone in the same skeletal mesh. I’d think it’d be connecting to some attached component, but I’m not sure how it would know what attached component to connect to.

I have some first person arms that are holding a weapon. The weapon is attached to the right hand socket. And I’d like the left hand to be controlled by IK to be at a specific socket in the weapon. And then I’ll set up a similar thing on the third person mesh.

There’s also the parent bone space option. I can’t even think of when I’d want to use that option.