Unicode is a standard for the consistent representation and handling of text expressed in most of the world's writing systems.

TPTQ Arabic fonts are Unicode compliant, so you need to enter your text correctly encoded. An easy way to find out if you have correctly encoded text is to copy-paste a sample of the text into Internet browser, for example into Google search.
Some old text documents working with Arabic or Indic languages use 8-bit encoding, which required to use proprietary fonts. Such fonts can only use 256 glyphs, which is not sufficient for correct rendering of Devanagari (or other 10 Indic writing scripts).
Just go to Glyphs, on the font presentation page.
LetterMeter, a Unicode based text analysis tool, helps compare multilingual texts and measure the frequency of particular glyphs. It has been moved to a different website: http://www.type-applications.com.
Unicode is an international computer standard for uniform representation of all the writing systems of the world. Unicode is the basis of most modern software internationalization.
TPTQ Arabic designs and develops Unicode-compliant fonts. Below are some instructions for using Unicode fonts to set Devanagari texts.
QuarkXPress® requires AXt fonts and Layout Ltd’s ArabicXT™ extension for setting Arabic text. AXt fonts are non-Unicode fonts, using the ‘Mac Roman’ encoding; Arabic glyphs are substituted for the Roman glyphs, so for example ‘alef’ replaces capital letter G.
The TrueType format was jointly developed by Apple and Microsoft in 1991, several years after the release of the PostScript Type 1 font format. Despite the format’s technical superiority (most of the system fonts on both Mac and Windows computers are TrueType) it never became popular amongst designers.
PostScript or Type 1 fonts were developed by Adobe in 1985 for use with their PostScript printers. Initially, this font technology was available only from Adobe.
Briefly, OpenType is a font format jointly developed by Adobe and Microsoft in the late 1990ʼs. It came into wider use only after 2000, when Adobe included support for advanced typographic features in their InDesign, PhotoShop, and Illustrator applications.