The hidden dimension of the urban morphology is the underlying the urban morphological rules system. The number of these rules has increased and their application tends to become more complex. The urban morphosis based digital approaches tends to become widespread. However, achieving the target values for all the rules is difficult. This impacts the social, environmental and aesthetic objectives of these rules. This paper proposes a classification of urban morphological rules to assist the digital morphosis of urban form. The aim is to endow the system of rules with a hierarchy, which can make efficient the automatic generation of the urban forms respectful of the urban law. Thus, this work promotes the concerns of artificial intelligence in urban morphology.