The most striking feature of the senior Shia clerical ranks is that there is no body that grants them: authority is recognised from below rather than conferred from above.
Recognition, not appointment
An ayatollah is a Twelver Shia jurist qualified to exercise ijtihad, independent reasoning from the sources of law, and a scholar comes to be called ayatollah when the seminaries and the scholarly community treat him as one. There is no examining board and no appointment. The grade above, grand ayatollah or marja al-taqlid, is defined by followers, since a marja is a source of emulation whom ordinary Shia choose to follow in matters of practice, and a marja with few followers is not much of a marja, which makes the structure genuinely bottom-up.
The dispute over clerical rule
The office is advisory and juridical by nature. The doctrine of wilayat al-faqih, guardianship of the jurist, developed by Ruhollah Khomeini and embedded in the Iranian constitution of 1979, gives a ranking jurist political rule, and that is a modern and internally contested position rather than the classical Shia view. Major jurists in Iraq and Iran have declined it, and the quietist tradition centred on Najaf holds that clerical rule is not the business of the jurist. The disagreement is doctrinal, current and consequential.