Picking mode is used to select and drag objects in the viewer.
The mesh faces must be visible on an object or region to allow it
to be picked. Enter picking mode by selecting the Single
Select 
  tool in a pull-down menu of the
viewer toolbar. If the Single Select 
  icon is already visible, you can simply click
the New Selection 
  icon.
You can also pick objects while still in viewing mode by holding down the Ctrl and Shift keys as you click in the viewer.
Use the mouse to select objects (for example, points and boundaries) from the viewer. When a number of objects overlap, the one closest to the camera is picked.
You can change the picking mode by selecting one of the toolbar icons:
  Single Select
  Box Select 
  Polygon Select
For details, see 3D Viewer Modes and Commands.