Invoice cum Bill of supply and mixed supply

Query 1:- invoice cum bill of supply is issued when the exempt and taxable goods is sold in the same invoice to unregistered person by the registered person, we can can issue invoice cum bill of supply to registered person by registered person?
Query 2:- for issuing invoice cum bill of supply exempt supply include nil rated and non gst goods supply?
Query 3:- if the exempt and taxable goods sold together  to unregistered person and registered person in single invoice then, we need to treat this as mixed supply as pe section 8, then we need to follow the mixed supply rule or invoice cum bill of supply?
Query 4 :- for issuing invoice cum bill of supply is Mandatory or optional?