For forms intended for double-sided printing, you can specify which side of the paper the master page is applied to: odd (front), even (back), no odd/even restrictions, or blank.
You can use master pages for the odd (front) and even (back) sides of a form intended for double-sided printing. For example, you may want the page numbers of a double-sided printed form to appear at the lower-right corner of the odd (front) printed pages and the lower-left corner of an even (back) printed pages.
See also 

To use master pages in a form intended for double-sided printing