Sources from the nineth--eleventh centuries distinguished four linguistic groups: (I) Parsi, the literary Middle Persian (mainly used by Zoroastrian priests). (2) Parsi proper (mutlaq), the literary and spoken Southern Early New Persian used from Fars to Sistan, which had retained numerous Middle Persian features and vocabulary, with relatively few Arabic loans. (3) In the North, (Piirsi-i) Dari: The term originally referred to the administrative and spoken Persian that had developed at the Sassanian court in Ctesiphon and was administratively used throughout the Empire. In Khorasan it had already replaced the local Parthian dialects even before I slamization.
As a ready vehicle for the Muslim administrations, besides Arabic, it became the vehicle of the Muslim mission into Central Asia and beyond, where a Persian variety had already been used as a lingua franca. This process not only led to the ultimate replacement of Sogdian, Bactrian, and Khwarezmian, but also inserted into this Dari an increasingly larger Arabic loan component as well as local eastern Persian and other I ranian vocabulary. (4) Pahlavi, Ar. jalt/avi, lit. 'Parthian': The term implied the non-Persian Iranian languages, particularly in western and central Iran which was once part of the former Parthian Empire.
In addition, Early New Persian varieties must also have been spoken in Sassanian border garrisons east and west. One of these was probably the ancestor offat Persian in the Sassanian outpost at the Caspian gate to the Caucasus, Darband. The others, probably older, were the outpost in Central Asia out of which ultimately developed Afghan and Tajik Persian. A major contributing factor to the Arabization of Persian was the magnitude of Arab settlement in Greater Iran, and presumably the intense interaction and intermarriage between the immigrants with the local populations at both the highest and lowest social levels, and after large numbers of Arab tribes moved into the Fertile Crescent during Sassanian times, Shapur II (r. 309-379) settled some of them in Fars as well as in the hinterland of Bam and Kerman. After Islam, settlement occurred in various waves throughout, and was most extensive in eastern Iran, including Khorasan, Afghanistan, and Central Asia. At its height, the number of Arab immigrants may have totaled 250,000. While these Arabic-speaking populations were ultimately absorbed, except for isolated Central Asian Arabic pockets (eastern I ran, northern Afghanistan, Central Uzbekistan), Arabic continued as the high register literary language during the earlier centuries of New Persian, mainly as the dominant language of science and religion, and may have at least indirectly affected even syntax, particularly through extensive translation activities both from and into Arabic.