What is a booking class (RBD)?
A booking class (or RBD, reservation booking designator) is the single-letter inventory bucket an airline uses to control how many seats it sells at each price level within a cabin.
An economy cabin might be split into many booking classes such as Y, B, M, H, K, L, Q, T and V. Each class maps to one or more fares, and the airline’s revenue management system opens and closes classes as demand changes.
Booking class versus cabin
The cabin is the physical product (economy, premium economy, business, first). The booking class is a pricing and inventory control. Two passengers sitting side by side can be in different booking classes and have paid very different amounts with different rules.
In booking systems
Availability responses list open booking classes and seat counts for each flight. A pricing request combines the chosen classes with eligible fares, identified by their fare basis codes.