What are these classes used for? Line, Arc, NurpSpline, and Ellipse

[EATREVITPOOPCAD]

Apologize in advance for the silly question but if in the Revit file your... 

2-D lines are these classes: 

DetailLine, DetailArc, DetailNurbSpline, DetailEllipse

and 3-D lines are these classes:

ModelLine, ModelArc, ModelNurbSpline, ModelEllipse

What are the classes below used for?

Line, Arc, NurbSpline, Ellipse

[RPTHOMAS108 · Accepted solution]

DetailLines and ModelLines etc. are elements deriving from CurveElement they contain geometry curves similar to how element geometry can contain solids.

i.e. geometry curves such as Line, Arc, NurbSpline, Ellipse are the underlying geometry of CurveElement instances along with other elements containing such curve geometry. A ModelLine and DetailLine both have a geometry curve which is a Line etc.

Line, Arc, NurbSpline derive from Curve and Curve derives from GeometryObject (as does Solid).

Reviewing the inheritance is the best way to understand i.e. ModelLine/DetailLine is to Element as Line is to Solid. They are on completely separate inheritance paths (one from Object->APIObject the other from Object->Element).