The difference can be studied on the basis of-:
Appointed- A sub-agent is appointed by an agent whereas a substituted agent maybe appointed by an agent or principle himself.
Responsibility-: A sub-agent is responsible to the original agent whereas a substituted-agent can be responsibe to the principle for all his acts.
Direction and control-: Likewise a sub-agent works under the instructions and control of the agent but a substituted agent works under the control of principle.
Privity of control-: A privity of control is made between the principle and the substituted agent, so both can sue each other. Whereas there is no privity of control between principle and the sub-agent.
Liability-: The agent is responsible to the principle for the acts of the sub agent but he is not made liable for the acts of the substituted agent, stating that he has taken proper care in choosing them.
Yes they are very much distinct in scope; sub agent has a wider scope than substituted agent.