It does not include the extra letters needed to write most Arabic-script languages other than Arabic itself (such as Persian, Urdu, etc.).
But in practice, and required for HTML and XML documents, ISO-8859-6 also stands for logical order text.
There is also ISO-8859-6-E which supposedly requires directionality to be explicitly specified with special control characters; this latter variant is in practice unused.
ISO-8859-6 was used as the reference standard for encoding the Arabic script in Unicode[4] but is now technologically obsolete.
The lower part of the character set differs from standard ISO 646 in the digits and in some punctuation.