Microsoft's Component Object Model (COM) was originally designed to support Object Linking and Embedding (OLE); it also became the basis of ActiveX Automation. As the standard for Windows component development, COM has relevance beyond OLE and ActiveX. Component architecture separates interface from implementation, allowing applications to consist of dynamically linked components rather than a single binary executable. Developers can write programs that take advantage of existing COM components, or they can use COM to create their own components.
ObjectARX applications can be designed as COM clients. For instance, an ObjectARX application that needs to communicate with another program could implement COM access to that program. Depending on the COM interfaces that the other application provides, the ObjectARX application could then exchange information with that application, or even drive it.
An ObjectARX application can also act as an Automation server. You can write COM wrappers to expose additional elements or custom ObjectARX objects. ObjectARX APIs, templates, classes, and support for the Microsoft Active Template Library (ATL) make it easy to add to the AutoCAD ActiveX Automation model.