Levels WindowKeyCreator \ Layout \ Overview \ Levels-Window
Levels should be used to organize and isolate the varying contents of a layout and to make the process of layout creation and/or modification easier. It is recommended that drawing instances be kept on separate levels for this very benefit. In complex layouts that contain several drawing instances, instances usually overlap one another. This will often cause for instance selection to be a more difficult task than it should be, especially if nothing was implemented to aid in the masking of these entities. By storing instances on isolated levels, this problem in particular, and others like it, can be alleviated. If this recommendation is followed strictly, you will be able to dictate what instances are to be displayed and when.
See View Level for more information.