If you see them on domestic segments, it's because the flight number for the domestic segment continues to an international (trans-oceanic) destination, usually in another aircraft.
Here are the codes:
J: Premium business class inventory; fares for a given destination usually cost significantly more than C class fares
C: Regular business class inventory;
Z: Discounted business class inventory; consolidators who sell discounted business class tickets on NW usually use this bucket; upgrade inventory for full "Y" fare, either from mileage-based upgrades or travel-agency incentive upgrades.
You'll be amazed how much the differences are between unpublished Z fares and regular C and J fares.
- Pat