I found this article on the Help file useful in comprehending the View Range's properties.
I'll answer those questions as well as I can:
@svalen wrote:
What is the difference between view depth and bottom?
Bottom keeps elements within the regular view range, and you can manipulate elements above the bottom plane. View Depth shows elements using the Beyond line style and you cannot manipulate elements above the View Depth and below the Bottom plane.
@svalen wrote:
What is underlay base/top level, how could you possibly look down from level 1 to level 2?
The top level is the top range of the underlay and the base level is the bottom range. Items in this range, meaning between the base level and the top level, will show with the underlay turned on, while elements outside this range will not show in the underlay. Look down means the underlay will use a Floor Plan-style view (meaning you will look down on the underlay, similar to a floor plan), while Look Up means the underlay will use a reflected ceiling plan-style view (meaning you will look up from the base level to the top level, similar to an RCP).
@svalen wrote:
Does underlay control if you can activate an object at all?..
There is a toggle for this on the bottom-right part of your Revit window called Select Underlay Elements. If you toggle this to "active", you can select elements shown in an underlay; if you toggle it to "inactive", you cannot.
@svalen wrote:
How do view range and underlay react to one another?
This I'm least confident about, but I believe they are mutually exclusive of each other, and the View Range governs. By this I mean if an element is visible because it is within the view of your plan, it will not be included in the underlay. For example, if you are viewing a floor plan, then items between the Cut Plane and the Bottom plane will not show in the underlay at all, but will be directly part of the active view. The relationship of elements to an underlay is a simple yes/no question--elements are either shown in the underlay in their entirety or they are not.
