Electromask / Mann files or sometime also called PAT files are used as input format for pattern generators for photo mask production. These pattern generator often work with a mechanical aperture and can expose a rectangular shape on a mask with a single flash. So a Electromask file consists of a plenty of rectangular flash information. No hierarchical structure is possible in these files. The encoding is often IBM-Ebcdic, rarely Ascii is used.
A Electromask stores only a single layer of a design. For this reason they are handled in a different way than multy layer file formats. With the open function a new cell is opened and the file data is added to the active layer. With the import function the file data is added to the current cell on the active layer. An update will replace all elements on the active layer in the current cell with the file data.
Electromask files only contain rectangulars in any orientation. All other polygons are ignored while saving to disk. Always all layers in the current design are stored. Use ExtractLayer and Fracture before to prepare the data.