Fare class Z is a business fare tier on many airlines. F or J is the top tier (F means/meant First Class; most airlines don't use this class name any longer.) Z is a discounted business class fare tier. Still pricey, but less than paying F or J fares.
Delta Fare class Z is a revenue fare/booking class of service that is marketed as Discount First Class / Delta One on Delta Air Lines. The fare class is considered a discounted First Class / Delta One.
When airlines introduced devotes business class cabins with much better seating, J was used to designate them. They were seen as a premium business class, as opposed to the fancy economy class (same Y seats, just better meals and usually and empty seat).
Delta-marketed, Air France-operated flights: Economy seats booked in Y, B, M, H, Q, K, L, U, T, X or V class can be upgraded to A class (premium economy), or from W, S or A class to Z class (business class), or from P, A or G class to Z class (business class).