The occulting requirement refers to the ability of the visual to display a minimum of 10 objects such as buildings or signs placed behind each other in a scene in the correct order, without one behind bleeding through one in front. Primarily an issue for older visuals, modern ones like CAE Tropos use Z-buffering. The older systems like Vital and Maxvue use occulting planes, which have to be added manually into every scene by the modeller.
Deferral or rather "to defer" is a fancy word for procrastination
