In the View Range dialog, levels used to define the visible range are absolute to the current level of the view. For example, if you are in the Level 2 plan view of a multi-story building and you select Level 4 as the top, Revit keeps Level 4 as the top, even if you add levels between Level 2 and Level 4. If you were to delete Level 4, then the clip plane would revert back to the default level with which the view is associated. In this example, it would revert to Associated Level (Level 2). Values, such as Level Above and Level Below, always have a specific level name associated with them. For example, if you are setting the view range for Level 1, the Level Above value would read Level Above (Level 2).