By default, all conducting and insulating layers are assumed to stack in layer table order, starting at the substrate. We therefor ignore the upper and lower conductor references in Via layers. Any layer order is acceptable, there is no constraint regarding adjacency of layer types, but of course the layer table order must match the physical order.
However, we allow for the case that Via layers are out of sequence in the layer table. They will be moved to the correct position in the stacking order used by the interface. Since layers are rendered bottom-up in drawing windows, having a Via layer positioned above the referenced top conductor in the layer table would cause the via to be drawn on top of the metal, probably enhancing visibility. The correct sequence, of course, would place the Via layer below the top referenced conductor.
The LayerReorderMode variable can be set to allow Via layer repositioning. The variable is set to an integer in the range 0-2.
Once the layers are recognized and sequenced, a three-dimensional representation of the layers found within a specified area is constructed. This representation is then available for such useful things as displaying a cross section, or building up an input file for a parameter extraction program.