Jack den Haan Mar 20, 2010:

Plug-in module versus expansion module The first generation means for connecting a notebook computer to the Internet through a GSM connection, for example, is a typical example of a hardware plug-in module. Many notebooks had and even now have special slots for that kind of module. In my opinion, you wouldn't call that an expansion module, because you would pull it out again after using it. As I see it, the latter is a more less permanent extension to, or indeed an expansion of, an existing hardware setup.