1) Bayt al-Māl al-khāss: This was the "royal treasury" or the privy purse, with its own sources of income and items of expenditure. It would cover the personal expenses of the Caliph, his palaces, harem, pensions of the members of the royal family, palace guards and gifts from the Caliphs to foreign princes.
2) Bayt al-Māl was similar to a state bank for the Muslims. According to M. A. Mannan this does not mean that it had all the functions of the present day central bank, but that whichever of these functions did exist in their primitive forms were performed by it. Administration of Bayt al-Māl was always in the hand of one person.
At the provincial level, the utmost head of the Bayt al-Māl was the governor of the province. He was in charge of collection and administration of the revenue. The central Bayt al-Māl was situated at the capital of the state so that it could be under the direct control of the Caliph.
3) The last and most important type of Bayt al-Māl was also a public treasury and it is called Bayt-u Māl al-Muslimîn, or the treasury of the Muslims. In reality, it was not only for the Muslims; its function, M. Mannan states, included the welfare of all the citizens of the Islamic state regardless of their caste, color or creed.
The function of this Bayt al-Māl consisted of maintaining public works, roads, bridges, mosques, churches and the welfare and provision of the poor. This Bayt al-Māl was situated at the chief mosque and was administered by the Chief Qāḍī at the provincial level.6
5. Muhammad, Abdul Mannan. Islamic Economics: Theory and Practice (Britania: The Islamic Academy, Cambridge, 1986), p. 175.
6. Ibid. p. 176.