e-gate is a truly pre-registered system - you need to have your e-gate card activated by a DNRD officer. This works for entry and exit clearance.

Smart Gate is not really pre-registered: you go through a non-registered channel first through the same automated gates, and then that forms the basis of your registration which is then validated by an officer right there and then, then you pass through as normal. On subsequent visits it's the same except there's no validation. Smart Gate is only valid for entry clearance at the moment.