The following formula appears correct in that I want to the correct response only if the UPN column is populated and the Construction column cell 1 is Greenfield, one answer if construction column cell 2 is Brownfield then index the exists column to see if the closest parent already exists. if so return "existing" if not return "new" and if the UPN@row is blank then blank.
=IF(UPN@row <> "", IF({Cardinal Metadata - UPN & UPI List 'Construction'} = "Greenfield", "NEW", IF({Cardinal Metadata - UPN & UPI List 'Construction 2'} = "Brownfield", IF(INDEX([Closest Parent]@row, {Cardinal Metadata - UPN & UPI List 'Exists'}, 0), "Existing", "NEW"))), "")