Sets each geometry coordinate dimensionality to the specified value: either 2D or 3D. Default way of geometry representation is 3D, but you may want to suite it to the specific platform. In AutoCAD, for example, 2D geometry is treated differently than 3D in terms of entity types being produced - a curve string in 2D results in a Polyline (a.k.a. Lightweight or LW Polyline), while the same geometric type in 3D results in a 3D Polyline.
Please keep in mind that certain feature data stores support 3D geometry, while others don't. This is defined by the data store geometric capabilities.