The Map Legend section of the Display Options Configuration controls the display of a legend for vector data and / or raster data with palette colors. This control is for an onscreen legend visible when interacting with the map. See also Map Legend Element
Access the Map Legend options by selecting the 
Configuration button from the File Toolbar or Tools menu, and navigating to the Display Options section. This tool can also be accessed from the View Menu > Workspace Layout Options > Map Legend... When a legend is displayed on the map, quickly access Map Legend Options... from the right-click context menu.
No legend will be displayed on the main map window.
A legend will be displayed for vector features on the map.
A legend will be displayed for palette colors for loaded raster files with palettes.
A legend will be displayed including both vector features and raster palette colors.
By default the legend will display vertically, with one value per row. Specify additional columns to divide the legend horizontally.
When styling is controlled by Feature Types, specify the attribute value used to label the legend. If the layer is styled based on Attribute/ Name Values this will be overridden by the list of values. For more information see Vector Data Layer Options.
When this option is selected, the legend entries associated with vector data will be split into groups by layer group (if there are any) and by layer description.
Select this option to use a natural sort order for labels, rather than an strictly alphabetical sort. This will keep numeric values in order of quantity.
Example
The below labels will sort as follows
Alphabetical sort with the option unchecked:
Numeric or "natural" sort with this option selected:
Specify which feature types to include in the vector legend.
Select which layers to include in the map legend.
Specify which palette colors to use in the palette legend, and setup the palette. For more information on palette setup options see the Layer Options Palette tab.
Give the legend a title or header label.
Add footer text to bottom of the legend.
Customize the font used in the distance scale.
Customize the background color of the distance scale.
Specify where in the map frame the element will be displayed.
Use the slider to control the opacity of the element.
Specify the border style for swatches in the legend.
Move the slider to the left to make the map legend transparent.