It turns out that legacy systems are often interesting when you look at them. There’s an interesting summary of the various numbers you’d find on a boarding pass here.

The e-ticket system is an outgrowth of SABRE. There’s a whole system of locator numbers (six alphabetical characters, such as IXYDBY).

Then, perhaps more interestingly, are the document or e-ticket numbers.

On American airlines, a seat upgrade might look something like this: 001 0629 793337 (spaces added).

Let’s break it down:

• 001 is the Airline Code, in this case for American Airlines.
• 0629 is the form code. I’ve also seen 0635 for seat upgrades; according to a gate agent, anything 06XX is seat-related. Other forms include 02XX for checked bags and 04XX for same-day upgrades.
• 793337 is the serial number; the specific document in question.

A ticket looks like this: 001 2280 476034. Note that the form code has the form 2XXX.

Tickets booked through a third party, such as Costco Travel or Expedia, have a form code of the form 7XXX.

As an aside, the AA refunds request site needs these e-ticket numbers to request refunds.