There’s a generally accepted “rule” that a diorama must contain figures to qualify as such. If, for example, your A6M2 Zero is shown parked on a runway strip, along with a generous assortment of palm trees, sandbags, oil drums and spare parts, it is not considered a diorama, but merely a “base” for your Zero model. As soon as you add a couple of figures (let’s say a pilot talking to his mechanic), it is magically lifted to the status of a diorama.
The rationale for this dictum is that a diorama should tell a story, or at least depict an event. Thus, figures are needed.
I find this argument somewhat short-sighted. It’s quite possible to tell a story without figures. Consider the example of a 1:200 scale scene of a large aircraft (let’s say a B2 bomber) crash landing in a forest. Due to the scale, no pilot or crew would be visible through the cockpit canopy. And the forest would presumably be uninhabited, unless you count small animals (which would be difficult to model in 1:200 scale). This is a much more dramatic event than a pilot talking to his mechanic. Yet some would say the crash landing scene is not a diorama because it lacks figures.
There are many examples of great works of art which don’t depict people, yet still tell a compelling story. The science fiction masterpiece 2001: A Space Odyssey is famous for the “machine ballet” which opens the second act of the film. Director Stanley Kubrick devotes several minutes to showcasing elegant spacecraft gliding through the void to the strains of The Blue Danube. He makes a powerful point by contrasting the beauty and harmony of these machines with the stilted, shallow and dull interactions of the characters in the movie. This is storytelling at its finest.
So how did it come to be that all dioramas are supposed to have figures? I’ll venture that it was a lack of imagination. No-one could figure out (pun intended) how to tell a story without putting people in it, so someone decided to dumb things down by mandating an easy fix: thou shalt have figures in thy diorama. Like most easy fixes, this one is a bit problematic: it makes false assumptions about how a story should be told. In fact, it’s a lot like the characters in 2001: stilted, shallow, and dull.