When creating a new document you must decide whether to set it up for spread editing, or editing as single pages. It's not possible to change a document that’s been set up for spread editing to single pages (or vice versa); this choice can only be made when a document is created (although we may add this capability to switch between to a future version). With the present version of iStudio Publisher if you change your mind it's necessary to create a new document and copy and paste the content from the existing document to it.
The main reason for selecting Spread Editing would be to enable the creation of documents in which content runs “across the join” between facing pages, for example, brochures and magazines.
A second reason for selecting Spread Editing would be to achieve page symmetry between facing pages, including when adding automatic page numbers and page margins. Selecting Spread Editing changes the page margin settings from Left and Right to Inside and Outside, and also changes the master pages to include a double page spread for all of the intermediate document pages, which would, for example, enable you to position an automatic page number on the left side of all lefthand facing pages and on the right side of all righthand facing pages.
NOTE: Selecting Spread Editing (or not) will not change how a document prints out or exports to PDF file, which can be either as single pages (by choosing menu option File > Export as PDF) or as a booklet PDF (by choosing menu option File > Export as Booklet PDF). Both of these options are available with any document, irrespective of whether it’s been created for Spread Editing, or as single pages.