A means the extension is fon format font, which is handed down from the old 16-bit system. They are generally bitmap fonts, and different fonts must be produced for different font sizes. For example, the default Fixedsys font in the XP Notepad in the system belongs to this type of font. T represents a font with the extension ttf or ttc. ttc (TrueType font) is a collection of multiple ttf fonts. ttf fonts were developed by Microsoft. This font technology can realize the actual printing effect on the screen, and it can be enlarged infinitely without distortion. O (OpenType font) refers to a font file with an otf extension or an OpenType font file with a ttf extension. It is jointly developed by Microsoft and Apple. It is more precise than ttf font curves and is used for high quality printing. Both ttf and otf are currently mainstream font formats.