Do you mean external layouts?, they are just object setups, or scene designs. The external layouts has no events logic: In your “real” scene (the one with events) you can create objects from external layouts with the appropiate action.
An example can help: Have you played one of those shoot’em up games with ships or planes?, of course you noticed that the enemies come in groups, in custom formations. Each group in GD could be an external layout, already placed in a formation, all you have to do in the scene is spawn one of the external layouts every x seconds
To use them just create a new external layout, edit it, select the scene that contains the object that will be used by the external layout, place some objects on it. Finally you can spawn the external layout objects on the scene each time you use the action “All actions >> External layouts >> Create objects from an external layout”.