The diagram (previous post,) was prompted by yet another argument over what was a Tool, and what was a Utility, whether they were Plugins or Extensions, or both.
My argument is that everything loaded into Ruby (except the Core,) is an Extension.
Then 3 main types: Plugin, Library and Service.
Although I show 3 "kinds" of Plugins, in reality there is often overlap. A given plugin may have several, (if not all,) subtypes as features.