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When you select 2-sided for 2-sided in Standard copy, you can copy two original pages onto the both sides of a single sheet of paper.
Touch the right Function button to select the stapling side.
After selecting the stapling side and touch the OK button, you can select whether you use the preview screen. If you select ON, the preview screen is displayed so that you can check the orientation.
Note you only need to adjust the staple side if you want it to flip the back side of the page vertically. It will be horizontal (book format) by default.
Here is a document with more detailed instructions and screen shots: