An interesting issue came up in a game we were playing recently: Our party was on an airship and attacked by flying young dragons. My sorcerer cast a spell that triggered a wild magic surge and rolled a self-centered fireball (at 20,000 feet, whee!).
Here's the issue. Fireball says:
The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.
I've noticed that certain monsters have the "Siege Monster" feature which says:
Siege Monster. The [monster] deals double damage to objects and structures.
This suggests that objects and structures are legally different things and therefore Fireball would ignite an object but not a structure.
But then, perhaps a structure is a type of object? What is the nature/relationship of these two classifications of entities?
(For what it's worth, we ruled that the rope rigging was flammable because rope appears on the gear item table).