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

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

Some MDL applications insert their own submenus in the Applications menu in OpenCities Map's main menu bar. (The Applications menu appears in the menu bar only when an MDL application has inserted a submenu in it.) Furthermore, most MDL applications have their own key-ins. These key-ins may not be available until the MDL application to which they are related is loaded. For example, the BATCHPROCESS key-ins are not available until the batchprocess.ma is loaded, possibly with the MDL LOAD BATCHPROCESS key-in.