Datum

Introduction
In FreeCAD the word "Datum" is normally used to refer to auxiliary geometry in the PartDesign Workbench. These geometrical elements will not form part of the final Shape of the Body, but can be used as references and supports for sketches and other types of features.

The following correspond to elements derived from the class, itself derived from Part Feature:
 * PartDesign Point
 * PartDesign Line
 * PartDesign Plane
 * PartDesign CoordinateSystem

The following are derived directly from Part Feature:
 * PartDesign ShapeBinder
 * PartDesign SubShapeBinder

Usage

 * 1) Switch to the PartDesign Workbench.
 * 2) Press.
 * 3) Select the Origin of the Body, and make it visible by pressing the  bar in your keyboard.
 * 4) Press  to open the plane task panel.
 * 5) In the 3D view, click on one of the standard planes, for example, the XY plane. Then press  to close the task panel.
 * 6) Now in the tree view, select the newly created DatumPlane, and then press.
 * 7) Create a closed wire, and then use  to extrude the sketch, and create an initial solid.

Now you can move and rotate the DatumPlane as you wish, by changing its properties in the property editor, and the Sketch and Pad will follow its new Placement.

You can add other types of datums to be used with other sketches and features.