GUIorRender

As See On Random Tip!  (wrapped up into a simple gizmo)

GUI Input will be passed though and visible in the GUI.
or
RENDER input will only occur at render time.

NOTE: Rendering from inside Nuke uses the same GUI  process, so you will only see the change when it's on put on the farm or command line using -x

Random Tip: "$gui" in expressions will return 1 in the UI and 0 at render time. Great for enabling nodes only when rendering on the farm (use it in the disable knob or Switch node).