Vehicle (Tank, Car, Boat, Spaceship etc...) with a terrain covered base AND figure > Diorama Category
Vehicle (Tank, Car, Boat, Spaceship etc...) with a terrain covered base NO figure > Category placement subject to review by contest admins -
So having the commander peaking out of the turret makes it a diorama.
Yes, I know that's debatable - we just had to draw the line somewhere.
Now, if the tank was crashed into a telephone pole, a raccoon looking at it..etc.... then even though there isn't a 'figure' it does tell a story and would be a diorama - hence Category placement subject to review by contest admins.