captain blender rig

hello people!
i am just wondering something about captain blender rig,i think the the book is too mechanical,it’s a awesome learning ressource but tony does’t explain the meaning of actions,he just says extrduder here,put constraint here,so i just wondering what is the meaning of 4 bones we extrude along y axis,the pelvis,torso,upperbody and shoulder bone,i haven’t undertant too the meaning of the shoulderloc bone.