After Imposition Type (discussed in Section 1) come the fields that define the layout geometry of each side of the output sheet.
Layout Units can be specified to be cm, mm, points, or inches.
Page Width and Height are the width and height of the pages of the InDesign Source document.
Layout Columns and Rows define the page grid on the sheet. For example, for a Saddle Stitch booklet, there are 2 columns and 1 row on each side of a sheet. If you’re using Step and Repeat to do sheets of business cards, a sheet might have 2 columns and 4 or 5 rows.
Sheet Margin Left is space between the left edge of the output sheet and the left edge of the leftmost page image. Similarly for Sheet Margin Right, Sheet Margin Top, and Sheet Margin Bottom.
Horizontal Spacing is the desired distance between the columns of source pages (or cards or labels or whatever). Ignored if there is only 1 column. Similarly, Vertical Spacing is the distance between the rows of source pages. Ignored if there is only 1 row.
Sheet Width and Height are normally computed from rest of the values; but if you edit Sheet Width or Height, the related Margins will be adjusted accordingly. For example, in the pictured dialog, typing “8.5” for the Sheet Height will set the values of Sheet Margin Top and Sheet Margin Bottom to be “0.75”.
Under these sheet geometry fields we see self-explanatory buttons for “Save Layout…” and “Apply Layout…”, a summary of current Registration status, the copyright notice, and the ‘do it’ Impose button and Cancel button.