When you create a database or add space to a database, the space is divided into allocation units of 256 data pages. The first page in each allocation unit is the allocation page. Page 0 and all pages that are multiples of 256 are allocation pages.
The allocation page tracks space in each extent on the allocation unit by recording the object ID and index ID for the object that is stored on the extent, and the number of used and free pages. The allocation page also stores the page ID for the table or index’s OAM page.