Thai Airways has multiple fare options when purchasing a ticket. When booking economy, you’ll see that there are some differences between the SAVER, Value, and FULL FLEX fares. Neither of them provide any indication of whether you can choose your seat or not.
You’ll see that the primary differences are the miles you’ll earn, the amount of fees to change your flight and eligibility to upgrade your flight using miles.
To choose your seat, find your confirmation code or record locator (usually around six characters). Then go to www.thaiairways.com and click on the Manage or Manage booking link. Then enter in your confirmation code and last name
On the next screen, scroll down to find “Additional Services”. Click that button.
In the following screen, you’ll then see options to select your seat, select your meal, and/or purchase additional space for luggage.
Be sure to click the “Next” button in the following page to confirm your new seats:
Hope this helps.