this field determines the order in which the child pages of a particular page are listed in the page tree. Comparing the position of a page to that of its siblings will determine its position in the page tree, the higher the position number, the further down the pages appear in the tree. Comparing the position to pages that are not its siblings is meaningless.
On Server it used to be that the position was undefined by default, meaning that the sibling pages should be sorted alphabetically. When manually rearranging them, pages got assigned a position.
On Cloud I believe that every page has a position, and its position number seems to be somewhat random. I would be curious to find out what the logic behind the position numbers is.