Apple Type Services for Unicode Imaging
The Apple Type Services for Unicode Imaging (ATSUI) is the set of services for rendering Unicode-encoded text introduced in Mac OS 8.5 and carried forward into Mac OS X. It replaced the WorldScript engine for legacy encodings. == Obsolescence == ATSUI was replaced by a faster and modern Unicode imaging engine called Core Text in Mac OS X 10.5 (Leopard).
Source: Wikipedia — Apple Type Services for Unicode Imaging (CC BY-SA 4.0)