A device that is not build by the global “build” commands, but instead is build only when the user opens it and presses the build button within it (or some special key combination, shift+left mouse click on the device). Then, this device will store it’s input and only let go of it when told so by the user.
This is nice when working with terrain that is not supported by the layout generator, f.e. when using distortion on your terrain. You cache your current terrain in the device, then lock preview and open the layout generator. You now have a layout that is accurate and already rendered.
A very useful scenario is when you want to edit/add headwaters. I tried to do this but because the layout generator was busy rendering my terrain the UI was very slow and the resulting render was inaccurate. I had to export the current height map, import it and then lock the preview in order to edit the headwaters.