Sometimes an X prefix means that the route is really a coach without advance booking but sometimes it just means that that particular journey cuts out one or two deviations off the main line of route. It all depends on the operator.
Where I live just getting out of the village screws up the chance of any long journeys. If I was going to go any distance I would have to drive to somewhere like one of the Oxford park and rides and start the journey there.
Some villages are only served by X service buses. In Fife, Auchtertool is only served by the X26 Kirkcaldy-Glasgow, Largoward only by the X8 Kirkcaldy-St Andrews.