(a) A person may be admitted as an inpatient to a mental health facility for treatment of mental illness only as provided in this Chapter, except that a person may be transferred by the Department of Corrections pursuant to the Unified Code of Corrections. A person transferred by the Department of Corrections in this manner may be released only as provided in the Unified Code of Corrections.
     (b) No person who is diagnosed as a person with an intellectual disability or a person with a developmental disability may be admitted or transferred to a Department mental health facility or, any portion thereof, except as provided in this Chapter. However, the evaluation and placement of such persons shall be governed by Article II of Chapter 4 of this Code.