Detailed Drawings > Annotating the Drawing > Working with Datums > About Using Set References
  
About Using Set References
Before you can reference a datum plane or axis in a geometric tolerance or on geometry, you must set it. When you set a datum, its name is enclosed in a rectangle. Once you have set a datum, you can still use it in the usual way to create features and assemble parts.
When displaying set datum planes, keep the following in mind:
A set datum plane, attached to a dimension, appears in a drawing only when you show the dimension to which it is associated.
Reference datum planes do not display in a drawing unless they are perpendicular to the screen.