The overlay plugin supplies actions and triggers for interacting with overlays. See here for more info on creating overlays.
Causes an alert widget to show itself. The action allows you to change the title, message, and input color of the alert widget. If you want to change the duration or media. Edit the widget in the overlay editor.
Adds velocity to a wheel widget in an overlay.
Fires when a wheel has landed.
Wheels are not syncronized between browser instances. If for whatever reason you have two separate OBS sources pointed at the same overlay (Don't do this). This trigger will fire twice.
In practice you don't need to worry if you use the tools CastMate has for adding the overlay to OBS it won't create duplicate overlay sources.