Short answer: airlines divide each cabin into single-letter booking classes (RBDs) that control how many seats may be sold at each price level. Fares are filed against those letters with fare basis codes that encode the rules, and branded fares package rules and extras into named bundles like Basic or Flex. The seat is the same; the inventory bucket, rules and bundle are not.
The four layers of a fare
It helps to separate four concepts that travellers blur into "ticket class":
| Layer | What it is | Example |
|---|---|---|
| Cabin | The physical product: economy, premium economy, business, first | Economy |
| Booking class (RBD) | A single-letter inventory bucket within a cabin that controls availability | Y, B, M, K, Q |
| Fare basis code | A 1-8 character code attached to a filed fare that points to its price and rules | QNCOWIN |
| Branded fare / fare family | A named bundle of rules and extras marketed to the customer | Basic, Standard, Flex |
Every ticketed booking touches all four: the passenger sits in a cabin, was booked in an RBD, paid a fare with a fare basis, and, on most full-service carriers today, chose a branded bundle.
Booking classes and RBDs
The reservation booking designator, or RBD, is the letter you see in availability displays and in the segment line of a PNR. Because RBDs are single letters, there are at most 26 of them, and each airline assigns its own meanings beyond a few near-universal conventions (Y for full-fare economy, J or C for business, F for first). Travelport's fares documentation describes the RBD as the inventory control code that tells the reservation system which "bucket" a fare must be booked in.
Revenue management teams decide how many seats to open in each bucket on each flight. A quiet Tuesday departure might have cheap K and Q buckets wide open; the same route on a holiday Friday may only offer Y and B. When a fare "disappears", the fare usually still exists; the bucket it requires has sold out or been closed.
Fare basis codes and fare rules
Airlines file fares centrally, mostly through ATPCO (the Airline Tariff Publishing Company), which distributes them to GDSs and pricing engines. ATPCO states that hundreds of airlines publish fares through its systems for distribution to GDSs and third-party retailers. Each filed fare carries a fare basis code, 1 to 8 alphanumeric characters, that typically starts with the RBD letter and encodes attributes such as season, advance purchase or one-way/return in airline-specific shorthand.
The fare basis points to the fare rules: change and cancellation conditions, minimum and maximum stay, advance purchase, routing, combinability, penalties. When an agent "checks the rules" before a reissue, they are reading these filed categories. Two fares in the same RBD can carry different rules; the letter alone never tells the whole story, which matters enormously in refund and reissue work.
Fare families and branded fares
Fare families put a marketing layer over this machinery. Instead of exposing K14NR to the customer, the airline sells Basic, Standard and Flex, each bundling rules (changeable or not, refundable or not) with extras (bag, seat selection, miles earning). ATPCO's Branded Fares data lets airlines define these bundles and what each includes so that GDSs and APIs can display them consistently; Travelport documents them as a distinct data record alongside fares.
Low-cost carriers arrived at a similar place from the opposite direction: they start from a bare seat and sell everything else as ancillaries, a model covered in our guide to low cost carrier APIs.
Why the same seat prices differently
Put the layers together and the familiar mystery dissolves. The passenger who paid more probably booked later (cheap buckets closed), bought a more flexible brand, bought a return spanning different seasons, or bought through a channel with access to different fares. Availability is per-bucket and per-flight, fares are per-market with rules, and brands repackage both. Add currency of sale, point of sale rules and private fares, covered in GDS vs consolidator fares, and identical seats with different prices is the expected outcome, not an anomaly.
What agents see through APIs
What your platform can display depends on the depth of the connection:
- Availability and RBDs. A full GDS integration exposes bucket-level availability; many simpler retail APIs return only priced offers and hide the letters entirely.
- Fare basis and rules. GDS pricing responses include fare basis codes and structured or free-text rules. Aggregator APIs frequently summarise rules to a few booleans (refundable yes/no), which is convenient but lossy.
- Branded fares and upsells. Support varies by supplier and by carrier. If brand upsell matters to your margin, verify per-airline brand support in the specific API before committing.
- What nobody sees. No agency channel sees the revenue management logic itself: bucket opening rules, forecast demand and bid prices stay inside the airline. Under IATA's NDC standard, airlines increasingly return dynamically constructed offers, where the price may not correspond to a classic filed fare at all.
We map these gaps supplier by supplier during flight API integration projects, because a storefront that promises rule detail its API cannot deliver becomes a support problem after the first schedule change.
Practical rules for agency teams
- Quote the brand and the rules, never just the RBD letter; two fares in one bucket can differ
- Record the fare basis in your CRM at booking time; it is the key to every later rules lookup
- Treat "refundable" flags from summarising APIs as provisional and re-check filed rules before promising money back
- When a fare vanishes mid-booking, re-shop rather than force-book: the bucket closed and the old price is gone
- For reissues, remember the new booking must satisfy both the new fare's rules and its bucket availability
This article is general information about travel technology and online marketing. It is not legal, tax or financial advice, and advertising platform policies change often. Check the current policy documents and take professional advice for your own situation.