Clicking on this icon opens the following dialog box:
Here you can specify whether you want to insert a vertical or a horizontal guideline, as well as the X- or Y-coordinate respectively at which it is to be placed. With the popup you can set here too whether only the current page or all master pages are to be affected by the changes.
With [OK] the line will be inserted at the specified position and
the dialog closed. Clicking on the
More button sets the
guideline in place but the dialog will remain open so that you can
create further guidelines. As usual [Cancel] closes the dialog without
adopting the settings.