IBM DB2 is a relational model database server that can be made spatially aware and thus integrate with a number of GIS products. In this context, the DB2 tag relates to instances of DB2 that are spatially enabled. Once spatially enabled, DB2 can perform spatial functions in much the same way as other competing databases. There are two ways to do this: the DB2 Spatial Extender (free) and the DB2 Geodetic Extender (a priced add-on for the Enterprise Server Edition only).
IBM states (PDF link) that:
Spatial Extender treats the Earth as a flat map. It uses planimetric (flat-plane) geometry, which means that it approximates the round surface of the Earth by projecting it onto a flat plane. This projection causes distortions, which can vary across the extent of the data, but the distortions generally increase toward the edges of the projected region…Spatial Extender is best used for local and regional data sets that are well represented in projected coordinates, and for applications where location accuracy is not important.
Geodetic Extender treats the Earth as a globe. It uses a latitude and longitude coordinate system on an ellipsoidal Earth model. Geometric operations are precise, regardless of location…Geodetic Extender is best used for global data sets and applications that cover large areas on the Earth, where a single map projection cannot provide the accuracy required by the application.
The implications of the different spatial solutions in terms of accuracy is discussed in this question.