Experimental modular plugins provide a different way to load a part of the plugin only if another plugin is loaded: move corresponding classes and resources to a separate module and register it as a plugin content module which depends on the target plugin. This way it's possible to check at compile time that all code from the plugin which depends on an API of the target plugin is located in that separate module.

Given that modular plugins are experimental for now, it's recommended to migrate optional dependencies to use plugin content modules only for plugins in the IntelliJ monorepo.