Standard impositions use a static sequence of page indexes for the front side, and another static sequence of page numbers for the back side of each sheet. Usually the sequences are designed such that between the front side sequence and back side sequence all page indexes together represent the full number of pages imposed on a sheet. E.g. for a two up imposition, which has four pages per sheet; two on the front side, and two on the back side, the page indexes 1, 2, 3 and 4 will be used in an appropriate order.
For more information see the entry on