In short, it is a record of the fare, pricing, and payment information for the ticket. For example, if the itinerary is repriced, the PNR would not change, but a new TST would be created.
According to this 2009 Amadeus TST Training Manual,
A Transitional Stored Ticket (TST) is a record created to store information that is printed on a passenger ticket. The system creates the TST automatically after you make an itinerary pricing entry, FXP (FXB, FXA …) The TST is attached to the PNR and can be viewed as required. A maximum of ten TSTs can be stored per passenger.
When you enter a request to display a stored TST, the system combined information contained in the PNR and the tst.
The following information is stored in a TST only:
- International sales indicator
- Fare basis/Ticket designator
- Not-valid-before and not-valid-after dates
- Baggage allowance
- Fare including currency code
- Equivalent fare including currency code
- Tax including currency code, country tax code, and tax type code
- Net Fare (Net Remit Ticketing)
- Fare calculation
- Banker' selling rate
- Additional Banker' selling rate
- Payment restrictions
The following fare element information is stored both in the PNR and in the TST
- Ticketing carrier
- Tour code
- Form of payment
- Original issue/in exchange
As outlined in the Amadeus e-University blog:
A stored fare, also known as a Transitional Stored Ticket (TST), is a record of all fare information for a PNR. The TST is created after you make an entry to price and store the fare, or when you issue a ticket or invoice. When a TST is created a number is allocated to that specific TST which is completely unique and cannot be re-used within the same PNR. This number allows you to identify the TST for the entire life of the PNR.