A bench mark is a permanent mark established in a field to use as a reference point. A benchmark can be a concrete base in which an iron bar is fixed, indicating the exact place of the reference point.
While a Reduced level is the vertical distance between a survey point (Point of interest) and the adopted level datum.
Reduced level can also be defined as:
The elevation of point A relative to the Mean Sea Level is called the reduced level (RL) of A.