Work Plane, Sketching Plane and Datum Plane

In most CAD systems, expecially with pure solid modelers, the user sketches on the active Sketching Plane. think3 application has an extended concept of Sketching Plane called Work Plane, but Datum Planes are also used extensively.

The analogy between Sketching Plane and Work Plane is that the XY plane of the program's Work Plane coincides with the well known Sketching Plane.

Starting a new model, the Work Plane coincides with the absolute reference system which is a sort of center point of your infinite, 3D space. We call that position the World Reference System. Once you move the Work Plane, your input will be referred to the new position of the Work Plane. If you want to work on the World reference system, you need to set the Work Plane back to that position.

A common mistake is to confuse Work Plane and Datum Plane; the Datum Plane (see the Datum Plane command) is an "auxiliary plane" that appears in the model history. Usually you define a Datum Plane just to define a Plane that has to be parametrically controlled by, for instance, the distance from a planar face of a solid.

Then, if you need to sketch on the Datum Plane, the Work Plane has to be placed on the Datum Plane. Since the Datum Plane is controlled by its distance from the face of the solid, changing that dimension, the Datum Plane and the related Work Plane will be moved as well.