Bentley Descartes can be extended using MicroStation Development Libraries (MDL). Many parts of standard Bentley Descartes are actually developed using MDL.

An MDL application can only be used in Bentley Descartes. Bentley Descartes can be (and almost always is) operated with more than one MDL application loaded at a time. Bentley Descartes can manage multiple MDL applications simultaneously. In fact, there are no practical limitations placed on the number of MDL applications that can be loaded.