Admitted September 2nd 1992.
It is a system of iconographic classification, used in many databases, among them the Index of Christian art (can be consulted in the Library), for the description and recovery of information relative to the images.
Links[1] https://www.museunacional.cat/en/print/h-van-de-waal-iconclass-iconographic-classification-system-amsterdam-north-holland-pub-co-1973 [2] https://pleiades.cbuc.cat:2082/record=b1085201~S3*cat