You can use multiple methods to measure distance of two points in a drawing in AutoCAD Map 3D or Civil 3D. These methods may (properly) give different results - so you need to choose the right method suitable for the given situation.
Let us assume a DWG drawing of a map (here a thematic road map near Prague) with an assigned coordinate system, measured points with unequal elevations (Z) and an active osnap mode.
The most simple command - DIST (or MEASUREGEOM) - lists the spatial (3D) distance of both points - here: 2107.2164 m.
The mapping command for measuring distances - MAPDIST - respects the assigned coordinate system and measures the distance on the surface of the current geoid (Earth curvature) - here: 2101.8605 m.
The add-on command csct_hordist ("Distance in 2D") from Civil Tools lists the plain 2D distance of the given points, i.e. their distance in the plan view, in XY projection - this distance is the shortest one from the three, 2101.65 m.