RS building rigs from many types of modules. Modules are not only arm, foot, tail, etc.. but many variations of them. For example, you can choose the needed module of the foot for your character.
Modules can be included in the system very easy. Now the system has only basic modules, but it is will appear with no limits.
Every module has the ability to customize by the user. And more, you can create your own module and include to system. You can easily duplicate the existing module and make some improvements to your needs.