The legend is used to calculate the sum of length or area of a particular type measurement. For example in the following screenshot, the total for the Length type is added and displayed in the Legend grid at the top-right corner of Viewer panel. If you change the operator to Deduct in Add column, the sum shall calculated accordingly in the Legend grid.
To show the Legend grid, perform the following steps:
1. Select one
of the following options where the Legend grid must be docked (in Viewer panel) from the Legend
drop-down list 
on Legend
panel of the Markup ribbon.
· Legend Left: Dock the Legend grid at the top-left corner of the Viewer panel.
· Legend Free: Legend grid is free to be moved anywhere inside the Viewer panel.
· Legend Right: Dock the Legend grid at the top-right corner of the Viewer panel.
2. Click the Legend toggle button 
on the Viewer
toolbar if the Legend grid is not visible. For additional information,
see Legend.
3. Click the Properties button 
on the Legend
panel of the Markup ribbon to
the to view the legend properties.
Note: Click the
Delete button 
on the Legend panel of the Markup ribbon to close the Legend grid, but it does not delete
the Legend grid.