Page Repetition can be set by either selecting how many times to repeat each page or by selecting the Fill all positions option (the option Fill all positions is not available in case of repetition from beginning). If your imposition is two-sided pages for the front and back will always continue from the front of sheet 1 to the front of sheet 2, from the back of sheet 1 to the back of sheet 2.
For example if there are 2 positions on a sheet and repetition is set to three times the front of the first sheet will contain two pages 1 and the front of the second sheet will contain one page 1 and one page 3. The front of the third sheet will contain two pages 3. The back of the first sheet will contain two pages 2, the back of the second sheet will contain one page 2 and one page 4 and the third sheet will contain two pages 4.
By selecting "From beginning" pages are not repeated immediately after each other (page 1, page 1, page 1, page 2, page 2, page 2, page 3, page 3, ...), they are repeated from begin after all pages are used (page 1, page 2, page 3, ..., page 1, page 2, page 3, ...). Repeat mode from the beginning is automatically used for "Booklet" and "Calendar" imposition types and also with "Cut & Stack" option (here is the only one possible). This repeat mode is highlighted by From beg. label.
Note on the repetition on this palette and at the same time on the Pages Palette: If from beginning is active, when setting the repeat of a specific page (on the Pages Palette), this page is repeated immediately after each other according to this setting (and, of course, all pages are repeated from the beginning). If the repetition from the beginning is not active, all pages are repeated immediately after each other, but the number of repetitions on the Pages Palette takes precedence (the number of page repetitions is therefore not added or multiplied in this case).