I have been tinkering with some code to generate 3d shapes for quick iteration game design and debug drawing. In some searching I came across the procedural addon for OGRE V1 so I guess it will be in a similar veil to that. I was going to turn it into an addon for OgreNext but I wasn't sure weather it should be a component or plugin?
So, what is the difference between a plugin and component?
Components are not optional and provide specific functionality. If your project depends on a Component, it must link to it and include all the source code.
Plugins on the other hand can be optionally installed; and your project can either chose to handle the case when the plugin is not present, or or refuse to launch / crash etc.
Another difference is that plugins provide "generic" functionality; thus in theory you can have two plugins offering the same generic interface but they are different implementations. Like RenderSystems providing D3D11, OpenGL, Metal & Vulkan. They are all 4 different plugins that can be used interchangeably.
Though in practice, save for cases like RenderSystems, this is rarely the case because due to Hyrum's law a plugin is likely not going to be exchangeable with another plugin. PSX and N64 Emulators in the 2000's also used Plugin systems to offer different GPU/Sound/Input implementations, but after a decade they realized emulators and plugins would often end up very tied together and thus the plugin model was abandoned (note that Ogre came out in 1999, it's not a coincidence it went the Plugin model too).
Therefore often the distinction between Plugins and Components is almost non-existent except for how they're linked together (Components are just libraries linked by C++, plugins must be registered and dynamically loaded at runtime; unless using static builds in which case plugins must only be registered).
Even the render systems don't really need to be plugins definitely conditionally compiled in though.
Reason being your targeting a platform and that platform you will only want to support certain renderers and wouldn't want a user to swap out the render system in your deployment.