What is the meaning of 'category_id' in each annotation?

[hqhQAQ]

I notice that each annotation has a field named 'category_id', the total number of all unique 'category_id' is 40, consistently in 'train.json' and 'test.json'. I wonder whether 'category_id' is the id of the object part, but it seems that the original paper does not mention it.