how to define which kind of bone should put in which part of body?

I know that the FK solver is for the spine and head, the Inverse Kinematics is for the legs and feet, Forward Kinematics is for shoulders and arms and hands. Is it always the case? and the drawing direction of the bones matters? shock

Also, if animation the creature wihout legs, such as snake and caterpillar, how to build the skeleton to animate its movement?
o