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What Is A Bench Mark In A Topographic Map?

What does a Bench Mark mean in a Topographic Map? I need this fast. Pls Help!

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1. Benchmarks are an important feature you can find on topographic maps. A benchmark is a marker that was placed in the ground when the surveyors were out making the map. A surveyor could place a surveying instrument on a tripod above that benchmark, hang a plumb bob from the instrument to hang directly over the center of the brass disk, and know that they were in exactly the same place as everyone else who used that benchmark. Benchmarks are typically marked on topographic maps with their elevation.
2. Bench marks are points in the real world where the latitude and longitude (and usually the elevation) are recorded to a very, very high accuracy and precision. In the real world, these are marked by small brass discs set in concrete (or bedrock) that include certain information on them. Bench marks are necessary for accurate surveying by providing well-established starting points for measures and triangulation. Bench marks are shown on topographic maps with an X symbol, and sometimes an identifier, such as the National Geodetic Survey Permanent Identifier, which is a six alphanumeric character code in the form AB1234. On a topographic map it would look something like this: X AB1234 or sometimes just X BM (BM means "Bench Mark").
3. A Benchmark is a point that is of known altitude and location. It is a reference, a known location that the rest of the map (or a previous map) is based on. At a Benchmark you will normally see a brass or steel "button" with information about the group who surveyed the area. When making the map, benchmarks are used as locations where a GPS or other instrument is calibrated. And then measurements of the topography around it is based in relation to that point. Though sometimes benchmarks are used to validate mapping methods (like Laser Ranging from an aircraft).