Bigcommerce API - In Orders, what does the id represent?

In orders(https://developer.bigcommerce.com/api/stores/v2/orders), what does the id represent? Is that order_id? id is explained in documentation as "The ID of the order, a read-only value. Do not pass in PUT or POST."

However, if we look at id property under order product(https://developer.bigcommerce.com/api/objects/v2/order_product), there are id, order_id, and product_id.

I am confused what id represent in orders and order product.