To properly form a 3D volume, the surface represented by any STL files must be closed(no holes or reversed vector normal) and connected, where every edge is part of exactly two triangles, and not self-intersecting. Since the STL syntax does not enforce this property, it can be ignored for applications where the void does not matter. The missing surface only matters insofar as the software that slices the triangles requires it to ensure that the resulting 2D polygons are closed. Sometimes such software can be written to clean up small discrepancies by moving vertices that are close together so that they coincide. The results are not predictable, may be sufficient of the surface must be repaired using another program. Vector 3D printers require a clean .STL file and printing a bad data file will either fail to fill or may stop printing.