The others are correct. Z is the discounted Business class fare. Same onboard service as full Business Class, but there are fare restrictions such as a change fee plus difference in fare if you want to change the flight.

In United Airlines' ticketing system, the letter "Z" represents a discounted Business or First Class fare. While you are seated in the premium cabin and receive all the standard amenities—such as priority boarding, extra baggage allowance, and upgraded meals—the "Z" code indicates that the ticket was purchased at a lower price point than a full-fare "J" or "C" class ticket. This is often a non-refundable fare or one with significant change fees. For frequent flyers, "Z" is a highly desirable fare class because it provides a significant "fare class bonus" for earning MileagePlus miles and Premier Qualifying Points (PQP). Typically, a Z fare earns 100% of the flight miles as base miles plus a 50% bonus, totaling 150% credit. It is essentially a "Business-Lite" pricing tier that offers the full luxury of the cabin at a more competitive price, but with more restrictive ticket rules than the most expensive premium fares.