system_pluginsis a config setting in tiddlywebconfig.py that lists the plugins which are configured for the running TiddlyWeb server. Another setting twanager_plugins lists those plugins which add functionality to the twanager command line tool. Some plugins need to be listed in both.
The value of the config item is a Python list of strings. Each string is the name of a Python package or module located either in the instance directory or somewhere on Python's