The Arabic script is one script and several traditions, and the differences between them are not only questions of style. In the Maghreb and in al-Andalus a distinct family of hands developed, rounder and more sweeping than the eastern book hands, written with a pointed nib rather than a broad one, and it carried two differences of spelling that a reader coming from the east has to know: fa is pointed with one dot beneath instead of above, and qaf with one dot above instead of two. A manuscript copied in Fez and a manuscript copied in Cairo therefore do not use the same letters for the same sounds, and a page read with the wrong convention yields wrong words.
The pointing is not the only divergence. The Maghrebi tradition uses its own reckoning of the abjad numerals, in which sad, fa, qaf, sin, dad, za, ghayn and shin carry different values from the eastern ones, so a chronogram or a dated colophon has to be read according to the tradition that produced it. Conventions for the vowel signs, for the marks of Quranic division, and for the shapes taken by certain letters at the end of a word differ as well.
Other regional families exist beside these two. Nastaliq, developed in Iran for Persian and taken up for Urdu, hangs its words along a descending diagonal and is nearly unreadable to someone trained only on naskh. The hands of Ottoman Turkey, of Central Asia, of the Sudanic belt and of the Malay world each have their own settled habits. Describing the Arabic script without naming a tradition is therefore describing an abstraction, and dating or placing a manuscript begins with recognising which family its hand belongs to.