Champions that can do consistent damage for long periods of time are effective at applying Sustained Damage. Basically, it refers to having a quick succession of shots with a short amount of downtime between firing. Vivian is especially effective at this damage because she can fire much longer than anyone else in the game without reloading. Similarly to Poke Damage, sustained damage is not going to get consistent kills, though champions may have some bursty skills which will get kills. It is really good for destroying shields and constantly refreshing cauterize on enemies.