英国火车票为什么有些可以选座有些不能?求翻译

Departs:London Paddington at 20:48Train Operator:First Great WesternArrives:Oxford at 21:51Seats Reserved:Coach: * Seats: ***, ***, ***Ticket Type:Advance SingleRoute:Only valid on booked First Great Western services and required connecting First Great Western services.Key Terms and Conditions: Valid only on your chosen service. Not refundable. Changeable prior to date of travel for a fee. 去爱丁堡的可以选座位,牛津不行,都是***,请问一下这票子什么意思啊?