It seems the DBMS vendors have taken to slicing and dicing their packaging into more and confusing editions and models. Very few DBAs have the time or energy to examine all of the available packaging options and be sure that they chose the correct one in terms of its features, functionality, and pricing.

Let's pick on IBM. At a high level, there are five basic platform (or operating system) choices for DB2: mainframe, Linux/Unix/Windows, AS/400 (iSeries), VSE / VM, and PDA (Palm/PocketPC). Choose your operating system. If you chose anything other than Linux/Unix/Windows, you are basically done. However, the fun begins if you choose Linux/Unix/Windows.

You have to know which exact operating system you will be using. You also need to choose an edition. What exactly is an edition? On IBM's Web site, they list six DB2 editions -- and they have recently announced what amounts to a seventh in DB2 Express.

DB2 Workgroup Server Edition (WSE) is a multi-user, single host, Web-enabled database with included Java support, but without support for inter or intra partition parallelism. WSE fits most closely into the departmental DBMS architecture. Its registration limit is a maximum of 4 processors.

DB2 Workgroup Server Unlimited Edition (WSUE) has the same functionality, and the same registration limit of a maximum 4 processors, but a different pricing model. WSUE falls somewhere between a departmental and an enterprise DBMS architecture - for larger numbers of users.

DB2 UDB Enterprise Server Edition (ESE) is the highest level of DB2 database version with intra-partition parallelism support (the database engine can process SQL statement segments in parallel), and inter-partition parallelism support (process a query in parallel across all of the nodes). ESE has Partitioning and Clustering options as additional add-on features. This is the enterprise DB2, what used to be known as EEE.

DB2 UDB Personal Edition (PE) is a single user database engine ideal for deployment to PC based users. PE is IBM's personal architecture DBMS. 

DB2 UDB Developer's Edition (DE) is a low-cost package for a single application developer to design, build, and prototype applications for deployment on any of the DB2 client or server platforms. To save development costs, you can license DE instead of ESE, WSE, or WSUE. You cannot use DE for production work. 

DB2 UDB Personal Developer's Edition (PDE) enables a developer to design and build single user desktop applications. PDE is a lower cost development option for single user application development.

DB2 Express was announced in the first quarter of 2003, and IBM touts it as "a specially tailored database offering for the worldwide small and medium business (SMB) market, with focus on enterprises of size 100 to 1000 employees." This "edition" seems to be targeted at IBM's partners, encouraging them to build applications on top of DB2 Express, targeting the SMB market. But it isn't really that clear whether DB2 Express includes one of the above editions or whether it is a new edition altogether.

Even though I picked on IBM here, they are not alone; you will have this challenge regardless of the vendor. Good luck understanding and selecting your DBMS architecture.