There are two types of module within the Citadel system.
System modules are the modules that are provided as part of the source code package. The source code for these modules can be found in "citadel/modules/module_name" (where module_name is the name of the module).
The build system allows a user to create and plug in a new module in such a manner as to minimize the effect on the original source tree. A user module can be added by placing its source code in the "citadel/user_modules/module_name" directory (where modules name is the name of the module). For users of code prior to SVN revision 5578 or release version 7.22 will also need to alter the Makefile to build your module, however, there should be no need to alter anything else in the original source package.