I believe it means that no seats are available on that flight. There are no advanced seat assignments, or there systems is down? At least that is the description that it says on the screen!
For whatever reason, it seems to happen when I am looking for tickets for ORD-DEN on their 777 flights and when trying to book last minute flights for UALs discounts.
I have called UAL direct and been able to book with them and get an assigned seat, probably depends who you get. Be careful, if it is an internet only fare, by calling and booking that way, you could lose the fare!
That is my two cents worth based on my experiences! If others know different, please correct me!
DC-8 61/63/71 DC-9-30/50 MD-80/82/83 DC-10-10/30 MD-11 717 721/2 732/3/4/5/G/8/9 741/2/4 752 762/3 777 A306/319/20/33 AT