You must upload artwork so that the finished piece backs up correctly. On two sided work, the head of one side backs up to the head of the other side. The head of a page is defined by the top of the rendered proof and the bottom is called the foot.
For folded products, we create a mock-up prior to approving the job to ensure the artwork is set up correctly. On jobs where the orientation is not so obvious, please describe how it should back up in the order notes.