If AutoCAD is installed, OpenPlant PID searches for SHX fonts in AutoCAD’s Fonts directory (the 'Fonts' folder in the installation directory of the most recently used version of AutoCAD). In OpenPlant PID, specifying a paired SHX Bigfont is optional, and is recommended if you require characters that cannot be provided by the base SHX font.
Older SHX fonts only support up to 255 characters, and thus require a paired SHX Bigfont to support languages with more than 255 characters (e.g. SHX Unifonts are the current standard, and allow a single font (and font file) to contain characters in the entire Unicode range. You should use SHX fonts only when you must ensure AutoCAD compatibility, and TrueType fonts are not a viable option. OpenPlant PID supports AutoCAD’s font format (including normal SHX fonts, SHX Unifonts, and SHX Bigfonts).
