By default, a drag selection will select all components under the mouse. This can be a problem when viewing the scene in shaded mode since your selection may include components hidden from your view.
You can use Camera based selection to only select components with a clear line-of-sight to the camera. This ensures that you don’t accidentally select components you can’t see.
To select only unobstructed components
This is a global preference that turns on camera based selection for all of the transformation tools.
Maya only selects the components unobstructed by other components from the camera’s point of view.