Model 204 is a flexible, high-performance database with relational capabilities. It is a mature, reliable mainframe product whose performance, functionality and connectivity have been continually enhanced for over 30 years. Model 204 is a robust, reliable and continually evolving mainframe database. Its flexibility, rich functionality and interactive development environment have allowed an integrated application infrastructure to be developed with minimal staff. Model 204 relies on its own type of bitmap index, originally devised by Professor Patrick E. O'Neil, and also combines the use of hash table, B-tree, and record list technologies to optimize speed and efficiency of database access.
Although M204 is a pre-SQL (and pre-relational) database product, it is possible to manually map the files of an M204 database to approximate SQL equivalents and provide some limited SQL functionality using Model 204 SQL Server.
Model 204 is most commonly used in government and military applications. Among other things, it runs Australia's social security system (Centrelink). It is used commercially in the UK by Marks & Spencer. It is also used at the Ventura County Property Tax system in California, the Harris County, Texas, Justice Information Management System, and in the New York City Department of Education's Automate The Schools system. An informal list of past and present Model 204 users, compiled in 2010, identified more than 140 organizations worldwide.
Here is one link having explnation direcly from the Horses' mouth: http://www.rocketsoftware.com/m204/products/model204
Datacom, IDMS, Adabas, Oracle, FOCUS, NOMAD, TOTAL/SUPRA/ULTRA, Model 204, SQL/DS.