Online terms and conditions

[packetswitch]

Tickets booked online (which are reserved seats) have the following information supplied.

Where a reservation has been made a customer must travel on the train that they have reserved on.

If a Standard, First Class or Citygold reservation has been made a customer may travel in standard class on another train that day as long as the journey is similar and the replacement train is not fully reserved. A supplement fare may be applicable.

Now. I've done this a couple of times recently. But here are my two questions:

1 - what does 'similar' mean in 'as long as the journey is similar'?
2 - what is the supplement fare (i.e. how much), when does it apply, and where could a customer find out answers to how much and when?