In Sunni tradition, the title is exclusively used for Imam Al-Ghazali, due to his unprecedented role in integrating Sufism (the school that deals with esoteric meanings) with Shariah (school that deals with the jurisprudence).
In TwelverShia the title is awarder to scholars. It was originally applied as an honorific to leading scholars, but now the use indicates their status in the hierarchy of the learned, i.e. After Ayatollah. It is divided into two sub ranks, Hujjatu l-Islām wa l-Muslimīn ("Authority on Islam and Muslims") and Hujjatu l-Islām ("Authority on Islam"), given to middle-ranking clerics.