Unfortunately for us, until you install it, an OpenType font looks exactly like the old regular Windows TrueType fonts that do not work for us. That gives most programs access to this rich source of fonts.
The Mac is still based on AAT, but the Mac system of 2007 ("Leopard", or 10.5) added support for OpenType Arabic fonts in addition. This is called OpenType, and is an extension of their font format TrueType. The other format was made by Microsoft and the font maker Adobe. Virtually no-one outside Apple has made use of this format, advanced though it may be. Any font that uses the AAT system will work fine on any current Mac in all programs that do Arabic. One was made by Apple, they call it the AAT ("Apple Advanced Technology"). For that, it needs further instructions, and they come - you guessed it - in two varieties. In addition, the computer must be able to connect the characters into words. But being a Unicode font is not enough - that standard just identifies each character. As mentioned elsewhere, the Mac, Windows and Unix all today follow the "Unicode" standard, while older fonts used different systems and cannot work. "Old" includes both fonts made for the old Mac and for old versions of Windows. But it may be hard to find them and to distinguish OK fonts from those that do not work.
There are very many such fonts out there, both for free and commercially, several hundred. Windows and Unix fonts made in the OpenType format can be used by most programs on new Macs (since late 2007).Unfortunately, there are only a couple of places we can find such, but those two are very useful. Newer fonts made particularly for the Mac will work on all Macs and in all programs.2000, will not work on any current Mac (unless they were made by Apple itself).