Specifying a span allows you to
visualize data within the specified window or range of values. The span equals the difference between the minimum and maximum thumb values for the visible range on the range slider. It is also possible to have a span equal to zero (both thumbs on the slider are pushed close
together), and the range on the range slider is displayed as a single value. For example, if you have a layer for building floors and
set Span to 0 and Step Interval to 1, using Step forward 
or Step back 
on the slider progresses the slider by one (step
interval) and displays a single floor. However, if you change the
span to 3, stepping the thumbs by one (the step interval)
displays the data for the entire span, that is, for 4 floors
(the floor matching the maximum value, the floor for the minimum value and
the two floors in between). The basic query used to display the
range-enabled data, in this case, is to display data that is greater
than or equal to the minimum of the range and that is less than or
equal to the maximum of the range.
The range span is configured in the Current Range group on the Range tab. When range is enabled in the map, you can type a value to customize the span. You can also interactively modify the span value by dragging the minimum thumb on the range slider. The values on the slider dynamically update and correlate with the values on the Range tab. You can optionally lock the span; this prevents you from interactively changing the span from within the range slider.
Tip:
If the span value appears empty, check to see if either or both the minimum and maximum values are disabled. An arrow icon appears for the disabled state of both the minimum
and maximum 
ranges.