How the auto-rig works
The algorithm computes the model bounding box, then for each checked body part it derives a normalized offset (relative to the box), scales it by box width/height/depth and adds the box center. Symmetric parts (left/right) are placed together through the mirror plane. Chains (spine, tail, legs) are placed along their natural axis. Parent hierarchy is rebuilt from the template, so you can tweak any joint afterwards with the gizmo.
V4 saves position AND rotation, exports .bbmodel with real groups/pivots/rotations, and lets you regenerate the skeleton at any time from the checked parts.