The database structure results from the history of data compilation, the constraints of existing application software and mostly reflects the way the research using the database is viewed.
The database has been built not only to query information for one or a few stars, but mainly to study, in a general sense, open clusters. This includes data comparaisons, data elaboration as well as evaluation of cluster properties. The latter analysis relies basically on membership determination.
Therefore clusters, rather than stars, form the basic unit of the database, and each cluster has its own directory and contains the relevant data. The various logical steps of the cluster study and the necessity of starting from the original data justify the adopted structure.