A Knowledge Base is a set of models. Models are of a type out of Design (exaclty one of this type), Prototype (zero or more), and Production (zero or more). A Model has a set of properties applying to it. Some of the properties are very important, such as the external language to be generated by GeneXus Generators and also the kind of database engines or plain file table systems that are to be used. A production or prototype model always holds a (posibly outdated) copy of the Design model. When one of the former are acessed by the programmer, an impact analysis occurs, and impact reports and actions (database structure modifications and model updates) take place.