Who appoints Ahlul Hall Wal'Aqd ?in other words how to know who should be its members. What are their obligatory characteristics.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

The Ahlul-Hall wal-‘Aqd (the body that elects the caliph) are the influential commanders, scholars, leaders, politicians and key figures in a Muslim society (influential in terms of decision-making in the Muslim state and having the power and authority to elect and depose the supreme Muslim ruler).

According to Al-Maawardi :

“There are three requirements regarding those eligible to make the choice (of electing the caliph):

1.  Being of good moral character, including all its requirements;

2.  Possessing adequate knowledge by which one comprehends who has a right to the leadership of the Muslim state and the requirements that must be met in the leader;

3.  Being endowed with the insight and wisdom which will lead them to choose the person who is most fit for the Imamate and who is the most upright and knowledgeable with respect to the management of the offices of administration.” [Al-Ahkaam As-Sultaaniyyah]

However, there is no prescribed method of selecting the members of the Ahlul-Hall wal-‘Aqd. Some scholars held that they should be chosen according to the customs and traditions of the society provided that the requirements of the Sharee‘ah are fulfilled. Shaykh ‘Abd Ar-Rahmaan Al-Maydaani wrote:

“The qualities of those eligible to choose the Muslim caliph and hand the leadership of the Muslim state to him differ according to the social context and time. The criteria in choosing the members of this body differ from time to time, from one environment to another, and from one society to another. It is a matter susceptible to change according to developments and changes in people's lives, how educated and civilized they are, and the scope of their social contribution. Urban societies and villages are susceptible to significant social change; accordingly the criteria in choosing the members of the Ahlul-Hall wal-‘Aqd varies. Since it is left for Muslims to decide such a matter, they are entitled to set the mechanism and criteria for choosing the Ahlul-Hall wal-‘Aqd through which the supreme Muslim leadership is established and do whatever serves the best interest of Muslims … This may be done by electing the elite of a given community to serve as members of a regional and local body within their regions and then by each country choosing the elite from among them (those regional and local panels) and those elites from different parts of the Muslim land forming the main Ahlul-Hall wal-‘Aqd body to choose the Muslim caliph.” [Kawaashif Zuyoof fi Al-Mathaahib Al-Fikriyyah Al-Mu‘aasirah]

Allaah Knows best.