Booklets and 2-ups

In a 2-up simplex job, odd-numbered pages are printed on the left-hand side of the sheet and even-numbered pages are printed on the right-hand side. The pairs of pages are in numerical order, as in this 6-page example:

Sheets may be cut in half to produce single page half-size sheets or left whole to show, for example, a double-page spread of a table too wide for a single page.

In a Booklet, pages are usually printed in duplex with a pair of pages on each side. If the paper size selected for the destination printer were A4, for example, then up to four  'pages' re-sized to A5 (portrait) could be printed on each (landscape) sheet. The pages are paired in an order such that you just need to gather the printed sheets together and fold the sheaf of paper in half. For example, these 12-pages may be folded together like a newspaper:

In the special case of a two-page booklet, both pages would be printed beside each other on one side of a single sheet (in effect, 'simplex').

See also On the back option ('Action' page of the Field dialog).