By default, the AllowToggleSelectKey property is exCTRLKey, which indicates that the user can unselect/select an event by pressing the left mouse button and keeping the CTRL key down. The AllowToggleSelectKey property indicates the key to be used so the user can toggle a selected event. You can use the AllowSelectEvent property to change the key to allow the user select new events or you can prevent selecting any event using exDisallow value. The AllowSelectEventRect property indicates whether the user can select multiple events by dragging a rectangle. 

The LayoutEndChanging( exScheduleSelectionChange) event occurs once the user changed the selection ( of events ), in the schedule view. The Selectable property of the event indicates whether the event can be selected at runtime. The Selected property of the Event indicates whether the current event is selected or unselected. The SelectEventStyle property indicates the way the selected events are shown. The SelectEventColor property specifies the visual appearance of the selected event. The SelectEventTextColor property specifies the foreground color of the selected event.

For instance, you can set the AllowToggleSelectKey property on exDisallow which indicates that no toggle selection is allowed. If the AllowToggleSelectKey property on exLeftClick, the first click selects the event, the next click unselect the event, and so on.