ERC dials (valueParm) go in the body of the model, but any body part  can be coded to respond to them.  The valueParm just makes a dial appear, and can have limits, etc.  The individual body parts have to be coded, in the rotation or translation channels, refering to the valueParm.

Little_Dragon's examples show the valueParm coding (first), and then the coding that goes in the body part's channel (second).

ERC can also be automatic and invisible to the user.  This is how a JCM works.  The coding goes in the morph channel, but nothing needs to go in the joint's channel.  The code refers to the joint, and reacts when the appropriate channel is used.

John