The Institutions of State in the Khilafah by Hizb ut-Tahrir

Above were the contractual conditions necessary for the Khalifah to be appointed. No other conditions are obligatory. However, some conditions are preferable, whilst not being obligatory, if confirmed by Islamic evidences relating to them, or if they are listed under a rule that has been confirmed by a sound (Sahih) text. For a condition to be obligatory, it should have evidence which includes a decisive command to indicate that it is obligatory. If the evidence does not include a decisive command then the condition becomes one of preference. No evidence containing a decisive command has been found except for the seven conditions mentioned above. Therefore they alone constitute the contractual conditions. As for the other conditions, whereby a rule has been confirmed as sound, these would constitute conditions of preference only. This is like the stipulation that the Khalifah should be from Quraish, a Mujtahid or skilful in using weapons or the like, which have no decisive evidence.