The Case component dialog selects an envelope for maps presented on bar structures, plates/shells, or volumetric structures. Select a few load cases and either click 
(Select Case Component) or select Loads > Select Case Component to launch this dialog.
Choose the envelope (top or bottom) of a quantity picked for the selected load cases. The top or bottom envelope is presented for maps displayed on bar structures (only for maps on bars), plate/shell, or volumetric structures.