This is a survey monument vs the original post being a control point. Survey monuments are established on a larger scale.

When you need to reference a known elevation or XY coordinate, there is a network of survey monuments with known coordinates, usually installed and documented by governments. You reference a known location’s elevation and north x west coordinates and then use instrumentation to determine angle changes to transfer the known elevation to a series of other locations.

When you section off property or build things, like roads or buildings, you use this network of points and triangulate paths off of them to the job site by calculating angles and distances. Since you have to maintain line of sight while traversing the distance, you add points you can use to pivot on and travel great distances. These points are called ‘control points’ or ‘benchmarks’.

The main post pic is a control point. Your picture is a survey monument.