Question:

What are intervals and gaps?

Answer:

When a geometry feature is trimmed or extended, the portion(s) of the modified element which is visible and is being displayed using the Feature Definition settings is called the "interval". 

The portion(s) of the original geometry which is not visible is called the "gap". This is the representation of the location of original element before modification.  The gap will be displayed when you hover over the original element.

An interval element is ruled to the original geometry (or "gap" element), and this dependency can be viewed in the Civil Model tab in the Project Explorer: