There are lots of things people sit on that we would not categorize as chairs. For example if someone sits on the ground, Earth has not become a chair. Even if something's intended purpose is sitting, calling a car seat or a barstool a chair would be very unnatural. If someone were sitting on a desk, I would not say that it has ceased to be a desk nor that it is now a chair. At most I'd say a desk can be used in the same manner as a chair. Certainly I would not in general want an AI tasked with object recognition to label a desk as a chair. If your goal was to train an AI to identify places a human could sit, you'd presumably feed it different training data.