Map Legend Properties Dialog Box, General
A legend helps the user interpret the meaning of color, size, and marker type differences for map elements on a layer. By default, all layers that have associated rules display legend items in the first legend. You can create additional legends for a map and change the which items appear in each legend.
Use the Map Legend Properties dialog box to change display options for the appearance and layout of the selected map legend.
To change the contents or format of the items in the legend, use the corresponding Rules dialog box for the specific layer type, and change options on the Legend page. For more information, see How to: Change Map Legends, Color Scale, and Associated Rules (Report Builder 3.0 and SSRS).
Click the Expression (fx) button to change an expression that sets the value of the option.