The Apple Fonts Group is responsible for how their operating systems handle fonts.
Their most important work has been the TrueType system, built into the OS since System 7.0 in 1991, and soon licensed to Microsoft for inclusion in Windows.
Over the next few years the advanced layout system GX (with advanced typographic layout, character substitution and font morphing capabilities) was developed, launching in 1994. Linotype and Bitstream both announced GX fonts. Unfortunately the technology was greeted with little enthusiasm from their customers, probably due to its large memory requirements for the time (not to mention the typographic naiveté of the desktop generation).
Apple Fonts Group commissions fonts to exploit its new technologies: TrueType versions of Helvetica and Times Roman with excellent low-resolution characteristics accompanied TrueType’s launch; Skia showed off GX’s font morphing, and Hoefler Text made use of its automatic character substitution.
GX technology is no longer supported by Apple but the font portions have been re-used within Apple’s new Unicode rendering technology ATSUI (Apple Type Services for Unicode Imaging), which has been present in the System since OS version 8.5 (Aug 1998). The original GX font tables - added data in TrueType files - are renamed AAT (Apple Advanced Typography) tables and are used by ATSUI to control rendering and line layout of Unicode text in ATSUI-savvy applications.
The Apple Font Group website offers free TrueType tool software to help font developers adapt their fonts for Unicode support.
Apple also produces and distributes system fonts for Japanese, Chinese, Korean, Arabic, Hebrew, Cyrillic, Indic and Greek scripts, as well as variation encodings of the Roman fonts for Icelandic, Romanian, Turkish, Croatian and the Central European (Slavic) languages.