Probably the most widely used buttons on the status line are the selection modes and selection masks. The selection modes section gives you three different options for selecting objects in the view windows. From left to right, these are Select by Hierarchy, Select by Object Type, and Select by Component Type.
Once one of these options is toggled on by clicking it on the status line, the selection masks section will update to display the relevant selection types for that mode. Notice in Figure above that the middle selection mode button, Select by Object Type, is depressed.
Therefore, the selection masks section displays a collection of buttons that have icons representing different object types. In this case, the selection types available for Select by Object type are (from left to right) Handles, Joints, Curves, Surfaces, Deformers, Particles, Rendering Nodes, and miscellaneous objects.
These selection modes and masks can make selecting objects in the view window much easier when the scene becomes crowded. For example, your scene may have hundreds of surfaces very close to one another with a curve tucked among them.
Selecting that curve without also selecting a surface could be tricky. In such a case, you could click the Surfaces button in the selection masks section to disable surface selection. Now when you click the curve in the view window, you will be unable to select the surfaces, making it much easier to select just the curve.