Show and hides Trim Box of currently displayed page. Trim Box defines where the page should be cut after it is printed - usually the Trim Box is marked with Crop Marks.
If you close the dialog by clicking the icon in Tool Bar the Trim Box will still be shown (even for newly opened documents).
Trim Box is hidden after a document is imposed.
When Trim Box is shown, the "Original Trim Box" information shows the size of currently displayed page. When Trim Box is hidden, the size of the first page is shown.
The dimensions are shown in the chosen SCALE. This enables you to exactly set the SCALE so that you reach the desired dimensions.
You can use standard tools of Adobe Acrobat to set the Trim and Crop Boxes or other 3rd parties plug-ins can be used.