Overlays

DSK, downstream key, vMix Overlay.
Definitions are shared by the session. Each Mixing Unit keeps its own On-Air list, with no count limit. The source is a Scene or an Input.
Transition presets are session-wide too. A duration in frames uses the frame rate of the Mixing Unit that fires the transition.
vMix Overlay1–Overlay8, and the vMix XML overlay list, follow the first eight On-Air overlays of the current Mixing Unit. Later overlays are not on that surface.

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.