The concept of Generalization Per Category (GPO is formalized. It is shown that GPC imposes lattice structures on entity types and their subtypes. A high level application oriented data definition language based on the GPC is outlined which allows the system to derive general entity types and organize their instances. Users are freed from undue efforts in the design of databases which are about entity types with rich varieties and high populations. Effective browsing of these databases and efficient execution of frequent queries against them are achieved by using the lattice structures among the entity types and their subtypes.