When you open a file, the program tries to recognize the format based on file's extension. 3D-Win binary file is always recognized first regardless its extension. For all other files 3D-Win checks all extensions defined in format converters and uses format matching extension with the shortest name. This way you can define several format converters for same format and let the system use shortest name as a default.
Double extension is used for binary files. The first extension determines the file type and the second extension file format. This way the format extension can be used to register file type to the operating system for automatic opening. Type extension can then be used, for example, to filter file names in the file opening.
xy.tdw = vector file
po#.tdw = cross section, # = surface code
pi#.tdw = profile, # = surface code
ml.tdw = measurement line
mm#.tdw = terrain model, # = surface code
ne.tdw# = square grid, # = surface code
tg.tdw = road geometry
kr.tdw = sounding file
pj = file group
Additional information about extension handling can be found from format converter settings.