A DatumPlane represents a 3d surface with finite extents. It can be either a rectangle with arbitrary orientation, or a cylinder whose axis is parallel to the project z-axis. If a datum is visible in a plan or section view, it will be displayed as one or more curves. These curves are determined by the intersection of the datum surface with the cut plane of the view. By default, the extents of these curves reflect the 3d extents of the datum surface. If the surface is a plane, then the extents represent the projection of the surface onto the cut plane. This matters, for example, when viewing a datum plane, really a 3d rectangle, along one of its diagonals. The extents of the curve do not vary with the location of the view, because we use the projection of the rectangle and not the actual intersection. If the surface is a cylinder, then the extents reflect the actual intersection of the surface with the cut plane. In addition, the curves that represent a DatumPlane can be modified on a view specific basis. In this case, the ends of the curve no longer reflect the 3d extents of the datum.

System Object
Autodesk.Revit.DB Element
Autodesk.Revit.DB DatumPlane
Autodesk.Revit.DB Grid
Autodesk.Revit.DB Level
Autodesk.Revit.DB ReferencePlane