Here is an explanation of the messages you might see on CrossCountry on-board seat reservation displays:
Available: These seats are available and cannot be booked during the train's journey. Unless the train is fully booked beore the day of departure, there will always be a section of the train which is unreservable for the use of customers without a seat reservation.
Available, but reservable en-route [origin] to [destination]: These seats can be reserved during the train's journey. This means that the seat is currently available, but may be reserved by another passenger during the train's journey.
Reserved: Seats which are booked for part or all of the journey's duration will be shown as 'Reserved'. If the seat is only partly reserved, it will include origin and destination information, e.g. Reserved from Coventry.