Since I use several interfaces, I briefly list what the individual interfaces are used for.
The Interface for the communication with the Quest Controller. Used to send the controller Accept/Complete/Track… events. The controller should forward the events to the quest component. (See BP_QuestController)
Used to send a event from the quest selector widget to the parent (Quest Interface for example) to change the currently viewed quest.
Used to interact with all kinds of actors in the world.
Used to send open/close events to widgets.
It also has a event to determine if a widget needs the cursor visibility.
Almost all widgets that can be closed by the player are using this.
Used to find the correct widgets in a layout.