There isn't any difference. It's just a way of increasing the prices as seats start to fill, in the same way that the budget airlines do but without the need for complicated software to balance supply and demand.
The airlines use software that examines the state of bookings, for a particular flight, at midnight every day. The software considers how many days are left before the flight and how many empty seats are available, and then calculates the price to charge during the following day.
The rail companies adopt a simpler procedure. If there are, say, 150 'value advance' seats on a particular train, 30 might be placed into band A, 40 into band B and 80 into band C. The operator knows that the band A tickets will sell out first, followed by band B and band C. If there were only 120 seats allocated to 'value advance', they'd only advertise tickets in band B and C. If there were 80 (or fewer) tickets available, they'd all be placed into band C.
Chris