IRON puts objects onto a plane intended for a 2D drawing.
 
IRON moves or projects the selected points, xlines, rays, lines, circles, arcs, ellipses, 2D polylines, 2D and 3D splines onto the same plane parallel to the current UCS XY plane.
 
The desired elevation of the plane (Z coordinate in current UCS) may be defined by entering the Z value on the keyboard or by specifying a point that already has the right Z coordinate.
Default value is  Z = 0.
 
The translation or projection direction is perpendicular to the XY plane.
Z coordinates get modified; but all X and Y coordinates in current UCS are maintained.

The drawing shows six lines before and after getting ironed.  

Xlines, rays, and lines will not be changed if they are perpendicular to the XY plane;
circles, arcs, ellipses, 2D polylines and 2D splines will only be changed if they are parallel to the XY plane already;
oterwise IRON considers their current positions as being corresponding to your wishes ("in dubio pro reo").
This is also the reason why 3D polylines, 3D faces, meshes, regions, and other 3D objects are filtered out from selection.
(From the start, the "circle", "arc", "ellipse" and "pline" commands always create objects parallel to the current UCS XY plane. On the other hand, the "spline" command easily may create 3D splines by mistake.)
 
Objects with non-zero thickness are excluded from selection.
Objects on locked layers generally do not get modified.
IRON has no effect on hatches, dimensions, texts, and blocks.