Overlays

DSK, downstream key, vMix Overlay.
Up to eight per Mixing Unit. The source is a Scene or an Input.

They sit on top of Program. Overlay in the main window sets position and size; the desk toggles turn them on and off.
The Overlay window shows Program in real time, so it uses 1 slot of the video output destination window limit. Closing it frees the slot. Transitions are Cut or Fade.