I don't think they are 'stored' anywhere. They are defined by the user when they write the file, and the user can define the sizes (block how wide, how tall, how long), tank (same), it's the same with the hangars.
That's how you can make a tank, but shape it like an oval, texture it with windows and make it an office building. Or stack several tanks of varying shapes/sizes and make them look like an air traffic control tower.
It they were meshes, you couldn't do that.