Objects can be deselected one at a time by left clicking on the selected objects. A more powerful way to deselect items is to use the Select menu > Deselect command. This gives you access to most of the selection methods available on the Select menu (Pointer/Window, Intersecting Line, 3D Box, XY Plane, XZ Plane, YZ Plane, Groups, Frame Sections, Cable Properties, Tendon Properties, Area Sections, Solid Properties, Link Properties, Joint Constraints, Labels, All), except that now they are used to deselect rather than to select.
For example, assume that you want to select all of the objects in your model except for those in a particular XZ plane. Do this by first using the Select menu > Select > All command and then using the Select menu > Deselect > Coordinate Specification > Click Joint in XZ Plane command.