This is the first part of a study on the classification of phenomena. It starts by addressing the status of classification schemes among knowledge organization systems (KOSs), as some features of them have been overlooked in recent reviews of KOS types. It then considers the different dimensions implied in a KOS, which include: the observed phenomena, the cultural and disciplinary perspective under which they are treated, the features of documents carrying such treatment, the collections of such documents as managed in libraries, archives or museums, the information needs prompting to search and use these collections and the people experiencing such different information needs. Until now, most library classification schemes have given priority to the perspective dimension as they first list disciplines. However, an increasing number of voices are now considering the possibility of classification schemes giving priority to phenomena as advocated in the León Manifesto. Although these schemes first list phenomena as their main classes, they can as well express perspective or the other relevant dimensions that occur in a classified item. The independence of a phenomenon-based classification from the institutional divisions into disciplines contributes to giving knowledge organization a more proactive and influential role.