AAT For Indic Scripts
For Indic scripts, the only features that are necessary are glyph re-ordering and substitution. AAT supports both of these. As noted above, OpenType fonts for Indic scripts require AAT tables to be added before they will function properly on OS X. Note, however, that this applies only to software dependent upon the system support of OpenType. Programs which provide their own implementation of OpenType will render Indic properly with OpenType fonts. (They may, however, not render Indic fonts with AAT tables correctly.)
Mac OS X 10.5 ships with fonts for Devanagari, Gurmukhi, Gujarati, Thai, Tibetan, and Tamil. Fonts for other Indic scripts are available from third parties.
Read more about this topic: Apple Advanced Typography