Irani Zoroastrians are distinguished from Parsis by a thousand years and one migration, and by almost nothing doctrinal.
The Parsis left Iran after the Arab conquest. The Iranis are the descendants of those who did not, and, in the Indian usage, specifically of those who emigrated much later, in the nineteenth and early twentieth centuries, fleeing hardship and persecution under the Qajars. They settled chiefly in Bombay and Hyderabad, arriving in a country where a Zoroastrian community of great antiquity and considerable wealth was already established, and they did not simply merge with it.
The differences are ethnic, linguistic and legal rather than theological. Many Iranis speak Zoroastrian Dari, a distinct Iranian dialect, where Parsis speak Gujarati. Their customs and pronunciation retain the Iranian forms. A ruling in 1909 confirmed that they were not bound by the decisions of the Parsi Punchayet, so they fall outside the institution that governs Parsi communal life. And they follow the Iranian calendar, aligning with the Fasli reckoning against the Parsi Shahanshahi majority, so the calendar split, which looks like a doctrinal quarrel, maps exactly onto the two migrations.
There is no substantial doctrinal difference between the two communities. Both hold the Avesta, both honour the teaching of Zarathustra, both tend the fire, both live by good thoughts, good words and good deeds. Intermarriage between them is now common, and the distinction matters chiefly to the institutions rather than to the people in them.