A known dimension is a dimension for a single reference entity or between two reference entities. It is represented by the dimension symbol prefix kd#. In sketcher mode, when you create a relation, you can use known dimensions on the right-hand side. Unlike known dimensions, reference dimensions are dependent of the section geometry and require regeneration. Regeneration updates the section geometry, which updates the reference dimensions. Thus, carefully use reference dimensions in a relation.

If all the measured entities in a sketch are external to the sketch, a known dimension is created, even if you select the normal dimension type.