I think there is some confusion here. Z class is on the A345 longhaul. Because pax flying those flights may be connecting to or from other flights, the same booking class is made available on other flights. However Z cannot be booked for an itinerary that doesn't include an A345 longhaul flight. The alternative approach (used by some airlines) is to specify in the fare rules that Z (or whatever) is an upsell for certain flights, and the other connecting flights allowed in the fare book into another class. There are advantages and disadvantages of each approach.