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Choosing select existing event allows you to choose from events you have previously created by showing you a list of all your events. From this list you can then click to link this event on any of the events to link it to this target.
If the target you are linking to has been created as a specific target, i.e. there are specific event types which should be linked to demonstrate progress, then by default the list shows just the events of this type. You can always switch to displaying all events by clicking the show all events button.
