Tickets or e-tickets and seat reservations are not the same thing in Italy, and you won't always need both. A ticket or e-ticket is what you need to travel from one city to another on any train in Italy, and it shows a seat reserved for the type of class and fare purchased. A seat reservation is what grants you a specific seat on that train and it must be issued for travellng in conjunction with a Rail Pass. All high speed trains in Italy require to have a seat reserved for a Rail Pass or a ticket/e-ticket with its seat assigned.
In Regional trains passengers travel without seat reservations, that means there's no assigned seating. You run the risk of standing if the train is full, however.
When both a train ticket and a reservation are required, that means you're getting an assigned seat for the trip. Reservations are required on high-speed trains, overnight trains, and most long-haul routes.