This flag’s name does not have the right connotation: it is not materializing the current state, it is building it up incrementally instead of sending a full summary (to materialize means to take bodily form—e.g. a spirit or ghost). I would recommend proper names like

    sealed abstract class SubscriptionInitMode
    object SubscriptionInitMode {
      case object Summary extends SubscriptionInitMode
      case object Diffs extends SubscriptionInitMode
    }