At this point you should do your own logic to determine what you're trying to interact with -- for example by doing a new Raycast. Generally though, fallthrough means the event was not handled by anything. It lets you react to "clicks on nothing". You can set up your game camera with a UICamera as well, and this will let you interact with all colliders, not just those handled by the UI. It's really up to how you want to do your game interaction.