Short answer: Category 31 and Category 33 are the ATPCO fare rule categories where airlines file their voluntary change and voluntary refund rules as data. Category 31 sets when a ticket can be reissued and what it costs. Category 33 does the same for refunds. GDS tools read both to price a change, and airlines check against them when they audit.
Thanksgiving change requests are already arriving. Every one of them runs through two numbers that sit behind the fare: Category 31 and Category 33, where an airline files its rules for changing and refunding a ticket. When a reissue goes wrong, the debit memo that follows usually names one of them.
What are ATPCO fare rule categories?
Airlines file each fare with a set of numbered rule categories through ATPCO. Each category covers one condition: advance purchase is Category 5, minimum stay is Category 6, penalties are Category 16. ARC's Debit Memo Working Group adopted the same categories as the reasons airlines give for fare rule violations, so the numbers show up on memos as well as in fare displays.
ATPCO names three categories as the most common standards for changes: Category 16 for penalties, Category 31 for voluntary changes and Category 33 for voluntary refunds.
What does Category 31 cover?
Category 31 holds the airline's voluntary change rules as data. ARC describes it as the way to automate reissue provisions. It can set:
- Who the rule applies to. A specific passenger type, for example.
- When the reissue can happen. The timing the airline allows for the change.
- What it costs. The amount the airline charges for the change.
- Which fares to compare. Process tags in ATPCO's reissue table say whether the new ticket prices on historical fares or current fares.
A voluntary change is one the traveler asks for. A schedule change made by the airline is involuntary and follows the carrier's own policy.
What does Category 33 cover?
Category 33 does the same job for voluntary refunds. ARC's list says it defines who the refund applies to, when it can take place, the conditions to meet and the charges.
One case catches teams often. ARC's reason list uses Category 33 when a ticket was refundable at purchase and the traveler failed to cancel before the flight, so the value is gone.
How is Category 16 different?
Category 16 is text. ARC's list says it sits outside automation, so an agent reads the penalty and applies it by hand.
ATPCO's own history explains why that matters. Under Category 16, much was left to the agent's judgment, and the same rule was applied in different ways. When SAS moved its rules into Categories 31 and 33, ATPCO reports that its changes and refunds became more accurate straight away.
Two ways a change gets priced
Category 16 text
- Agent reads the penalty text
- Agent works out the fare difference
- Agent enters the reissue by hand
Category 31 data
- GDS reads the coded rule
- GDS prices the difference, taxes and fee
- Agent reviews and issues
The new ticket reaches IAR and the airline audits it against the filed rule
How do GDS tools use Category 31?
Automated exchange tools read Category 31 to price the change. Amadeus says its Ticket Changer Reissue works out the fare difference, the tax balance and the penalty from the Category 31 rules the airline filed. It then prepares the booking for reissue, and the agent issues the new ticket and any EMD.
Amadeus also says it guarantees tickets reissued under Category 31, and puts the time saved at 12 minutes a ticket from its internal data. Both are the company's own claims. Whichever GDS you use, check how its exchange tool treats a fare with Category 16 text alone.
Which debit memos come from these categories?
ATPCO says the most common reason for billing disputes between airlines and agencies is change and refund rules applied inconsistently. ARC's reason list shows where those disputes land:
| Reason on the memo | What happened |
|---|---|
| Voluntary change error, Category 31 | The exchange broke the filed change rule |
| Penalty fee not collected | The exchange went through with the penalty missing, including a penalty paid on an MCO left unlinked to the exchange |
| Penalty fee not accurately collected | The penalty was collected at the wrong amount |
| Refund of non-refundable fare | A non-refundable ticket was refunded with no airline approval on file |
| Voluntary refunds, Category 33 | The refund broke the filed refund rule |
When a memo carries several violations, ARC's FAQ says airlines usually name the largest one as the reason.
How should a team handle changes in a busy week?
- Price with the automated exchange first. Let the GDS read Category 31, and keep its pricing record with the ticket as your proof.
- Read the rule when the tool stops. A fare with Category 16 text needs a person to price it, and a second person to check it.
- Link every MCO. A penalty paid on a separate document belongs to the exchange in IAR.
- Check refundability before departure. Cancel refundable tickets before the flight to keep their value.
- Clear voids on time. Under ARC's holiday schedule, GDS tickets issued from November 25 to 29 can be voided until November 30.
Keep the proof with the ticket
If a memo arrives months later, the pricing record shows which rule the exchange followed. Store it with the PNR history and the waiver code, if one applied.
Common questions
What is the difference between Category 16 and Category 31?
Category 16 holds penalty text that an agent reads and applies by hand. Category 31 holds the same kind of rule as coded data, so a GDS can price the change automatically.
Does Category 31 apply to a schedule change?
Category 31 covers voluntary changes, the ones the traveler asks for. A change the airline makes is involuntary and follows the carrier's own involuntary policy.
What does Category 33 cover?
Voluntary refunds. It sets who the refund applies to, when it can happen, the conditions to meet and the charges, according to ARC's debit memo reason list.
Which debit memos come from Category 31 errors?
ARC's reason list includes a Category 31 voluntary change error, a penalty fee missed on an exchange and a penalty collected on an MCO that was left unlinked to the exchange.
Is an automated reissue always right?
It follows the data the airline filed. Amadeus says it guarantees tickets reissued under Category 31, so keep the pricing record with the ticket as your proof.
Sources
- ARC, Debit memo reasons (Debit Memo Working Group, updated March 14, 2016) ARC
- ATPCO, How to make change and refund billing disputes a thing of the past (January 13, 2021) Company's own claim
- ATPCO, Automated changes and refunds: foundational for airline retailing (April 19, 2022) Company's own claim
- Amadeus, Ticket Changer Reissue sales sheet (2024) Company's own claim
- ARC, Holiday schedule (2026) ARC
Checked on November 12, 2026. Rules change, so check the source before you act on a date or an amount.
