#Datums: planes, axes, points

A datum is auxiliary geometry to build from: a plane for a sketch, an axis for a revolve or an array, a point as a reference.

• Datum plane — a sketch does not have to lie on a face of the part. A plane offset from a base plane or from a face gives you a place where no face exists yet.
• Datum axis — an axis of revolution, of a circular array, of symmetry.
• Datum point — a reference for dimensions, the centre of an array, an anchor for a hole.

#Datums are parametric

A datum is given not by coordinates but by a definition: “20 mm off this face”, “an axis through these two points”. So it follows the geometry: change the part and the datum rebuilds, and with it everything that stood on it.

The offset takes a formula, like any numeric field.

#Why bother when there is a face

A sketch on a face is tied to that face. If the face disappears — eaten by a fillet, say — the sketch is left without support. A datum lives its own life and has no such dependency.

Second: a datum exists before any body is built. You start from one when there is nothing to build on yet.