In case anyone thinks that "Unavailable" appears only with respect to multi-leg journeys, that isn't true. Here's a non-stop example (LAS-MDW flights departing in a few days) where one flight with BS & AT flights available states "unavailable" and the next flight with BS & AT flights available states "sold out."



Note: "Unavailable" is also used for same-day flights with departures times in the past or less than 1 hour (or whatever the exact advance purchase requirement is for on-line flight purchases) in the future.