The term benchmark originates from the chiselled horizontal marks that surveyors made in stone structures, into which an angle-iron could be placed to form a "bench" for a levelling rod, thus ensuring that a leveling rod could be accurately repositioned in the same place in the future. These marks were usually indicated with a chiselled arrow below the horizontal line.
The term is generally applied to any item used to mark a point as an elevation reference eg. cut marks, rivets, pivots, bolts, flush brackets, projecting brackets and fundamental benchmarks.
The height of a benchmark is calculated relative to the heights of nearby benchmarks in a network extending from a fundamental benchmark. A fundamental benchmark is a point with a precisely known relationship to the level datum of the area, typically mean sea level. The position and height of each benchmark is shown on large-scale maps.
The terms "height" and "elevation" are often used interchangeably, but in many jurisdictions they have specific meanings; "height" commonly refers to a local or relative difference in the vertical (such as the height of a building), whereas "elevation" refers to the difference from a nominated reference surface (such as sea-level, or a mathematical/geodetic model that approximates the sea level known as the geoid). Elevation may be specified as normal height (above a reference ellipsoid), orthometric height, or dynamic height.
Info courtesy of Thejackrustles.
The cache is placed directly above the benchmark.