Plug-ins are the core of RealWorld Designer. Almost everything that could have been designed as plug-in, is a plug-in, usually following the cooperating plug-ins design pattern. The reason is simple - flexibility and power. It is very seldom necessary to modify existing code, addition of new component and reconfiguration is usually sufficient.
Some plug-ins are installed by default, others are downloadable on the Online page within the applications.
Plug-in is a COM class placed in a COM category. Each type of plug-in has an associated COM category. One COM class can be placed in multiple categories.