since about two year, Thai offers restrictions concerning advanced seat reservations to the lowest booking classes V and W - they both have access to rows 50 and beyond, and on the A380 its limited to Maindeck starting from row 50 only. On Upperdeck Thai offers a 2-4-2 abreast (instead of 3-4-3) and, therefore, these seats are first offered to those passengers who are willing to pay a little bit more. All other passengers will have access to Upperdeck only by (online) checkin (if Upperdeck is open for sales at all, because on some less demanded flights they try to save costs and staff and dont offer the Upperdeck Economy compartment at all).