Use to mark the current card or return to previously marked card. A dialog box pops up to ask you to choose "Mark" or "Return". Hold the Option key down while clicking this button to mark the current card AND return to the previous marked card. By marking a card, you can return to it quickly from anywhere in these stacks. Only 1 card can be marked at a time. When you select a Nutshell cross reference in the "Command" stack or use the squirrel icon, the current card is automatically marked.