Plugins usually add global-level functionality to VuePress. There is no strictly defined scope for a plugin - there are typically several types of plugins:

Extend the page's metadata generated at compile time. e.g. @vuepress/plugin-last-updated;
Generate extra files before or after compilation. e.g. @vuepress/plugin-pwa;
Inject global UI. e.g. @vuepress/plugin-back-to-top;
Extend the CLI with custom commands. e.g. vuepress-plugin-export.

Here is also a litte slightly complicated plugin example @vuepress/plugin-blog. that uses compile-time metadata to generate some dynamic modules and initialize them on the client side by using enhanceAppFiles.

The architecture of the whole plugin system is as follows: