When are the IO and PO appointed?

When are the IO and PO appointed? IO and PO are to be appointed if there is a need to inquire into the charges. The need will emerge only when the Charged Officer denies the charges or does not respond to the charge sheet. Thus the appointment of IO PO will arise only after the 91 charge sheet is issued and the charged officer either does not respond to it or denies the charge without convincing the Disciplinary Authority