What is the difference between reserved and unreserved train tickets in Sri Lanka?

Reserved tickets set aside one exact seat number for the full trip. Unreserved tickets allow you to board, but you do not get an assigned seat.

With an unreserved ticket, you scramble when you get on. You share the limited space with local passengers and other visitors. If every carriage is already packed, you may end up standing in the aisles. Sometimes you sit close to the open door area, if there is room. Carriages with reserved seating are watched by conductors. They check tickets before letting passengers in. Only people with the right tickets can enter those carriages.