What exactly spine is in UIPageViewController?

Question

The iOS doc says that we can present one or 2 pages at a time depending on the spine location or position and double sided state...
But, I am unable to understand "What exactly spine is in UIPageViewController ?"

Answer 1

It applies only when transitionStyle is set to PageCurl (otherwise it evaluates to None); during the animation of the UIPageViewController a 3D page is displayed: this property controls where the border of the page is attached with respect to the parent controller (that is, the position of the 'spine' of the displayed 3D book).

Min means it is attached on the left side, Max for the right, and Mid displays two consecutive pages split in the middle. In pictures (the spine is in red):

Reference: Apple documentation.