The Defpoints Layer is a standard system layer used by AutoCAD and IntelliCAD.
When you create dimensions in IntelliCAD or AutoCAD, the dimensioning components such as extension lines, arrowheads, dimension lines, and dimension text are placed on the current layer. The first time dimensions are added to a drawing, the software also creates a special layer named "DEFPOINTS". This layer holds the "definition points" for the dimension. By default this layer does not and cannot be made to print.
Definition points are the anchor or reference points for dimensions. They are in fact point entities that are drawn with and internally associated with each dimension drawn. You can see them clearly if you turn off all layers except DEFPOINTS.
Because these reference points can serve no purpose on hardcopy output, layer DEFPOINTS does not print under any circumstances.
The catch is, while DEFPOINTS is created automatically, you can easily set it to the current layer and draw on it just as you would any other layer. However, nothing you draw on this layer can ever be printed out. The unwary can easily be caught as entities display in the drawing editor, yet cannot be made to print out under any circumstances.
If entities are drawn on this layer by mistake they must be moved to a controllable layer to allow them to print. Alternatively, layer DEFPOINTS can be renamed to any other name. IntelliCAD will create a new DEFPOINTS layer with any subsequent dimensions.
As noted above this layer holds the "definition points" for dimensions and therefore placing other objects on this layer will significantly increase the possibility accidently moving these "definition points" and corrupting dimensions in the drawing..
On a positive note, DEFPOINTS can be used to hold all manner of information that you will never require to be printed. While this is useful at times, the assignment of one layer to dual purposes, will eventually prove unsatisfactory and defeats the true role of layers as a graphic data filter.