My guess would be "travel agent purchase, preferred seat, or "non 988 ticket number." These are the restrictions on the site:

• Passengers who meet the following fare conditions among passengers who have completed their purchase of an economy class flight ticket on Asiana Airlines’ regular flights
• This service is only available for customers with Asiana Airlines flight tickets starting with “988” and not available for customers with other airline tickets
• Restricted passengers: Passengers who travel on a codeshare flight or preferred seats, who travel with infants or children, who requested special meals, who travel with pets (on board), and who use Unaccompanied Minor (UM), Cabin Baggage (CBBG), or Extra Seat (EXST) service * Group bookings purchased via travel agencies are restricted from purchasing this service.
  
  * This service is not available to purchase for customers who are eligible to purchase but have already completed online check-in. Customers who are willing to purchase this service but have already completed online check-in should cancel their check-in first, in order to make the purchase.