Constrained against a fixed world direction.
Constrained against a direction expressed in the host bone's frame, where +Y is the host
bone's own direction. (fabrik3's equivalent uses +Z, so a rig ported between the two needs
its axes rotated.)
Constrained against the host bone's direction, so the chain bends relative to what it hangs off.
The first bone may point anywhere.
How the first bone in a chain is constrained.
The first bone has no bone before it, so it is constrained against a direction held on the chain. The
LOCAL_types only mean anything for a chain in a Structure2, where solveStructure resolves them against the bone the chain hangs off.