The PAGEINFO line specifies information required to place the page on the correct side. The examples in the Page Descriptions section show that the first number (1) is the page order number for a simplex page, the second number (1) is the page order number for a duplex page, and the third number (0) indicates to which side (0 = first side, 1 = second side) this page description applies.
If a page is intended to be for the second side in duplex mode, then the first number must be zero.
See INVERT for more information on pages.