When a 3D model is exported to a 2D file (in this case DXF or SVG file formats), one of the dimensions has to be ‘removed’. This is called a projection in mathematics. There are two main options during export for which projection is used. The first, is a global projection on one of the axes planes, and the second is a per-plane projection (suited to Surface Draw or Outline objects that are drawn on Plane objects).
The ‘flattened to’ option controls what projection the 2D export uses when flattening the 3D model. The XY plane option will cause all Z values to be output as zero, the YZ plane means all X values are zero, and the XZ plane means all Y values are zero (see Importing and Setting Up Coordinate Systems). If the data is not 2D already, or the objects are not drawn on a plane parallel to the projection plane, there may be distortion in the result. The impact of this can be reviewed on the Flatten Report).
The special ‘Surface planes’ option can only be used with Surface Draw or Outline items on Plane Surfaces. When this option is used the Items tab will not apply and the only items exported will be Surface Draw or Outline items that are associated with Plane Surfaces. Each individual plane surface will be used as separate ‘flatten to’ plane. That is, there are multiple projection planes.
Each of these flattened sections are placed in the export file with their relative positions somewhat arbitrary. To help identify which parts of the file are created by different surfaces you can use the Color per Surface and/or Layer per Surface options. These will color (and layer) the Surface Draw lines/curves and Outline items based on which surface was used for flattening. Use this option when you have surface draw on planes that do not match one of the axis planes (xy, yz, or xz).
This means, the objects of the same color (or layer) came from one Plane Surface and will have the correct geometric relationship, but objects of different colors (or layers) will not have a consistent or predictable relationship.
The main purpose of this export is to take a curved surface, that has been modeled by multiple plane approximations, to be exported with minimal distortion. Although the Surface Draw or Outlines will be flattened and exported without distortion within each Plane, the same is not true between Plane surfaces. You cannot count on the distance between two Plane surfaces in the 3D model being the same distance in the 2D export (with per surface flattening). When these surfaces are moved to the xy plane, the center of each surface is projected onto the xy plane and then the data is rotated. Depending on the angle of the surface to the xy plane, the distance between plane surfaces can change a lot. Beware of drawing entities between surfaces when using per surface flattening.