Monterey's README.MD file defines Monterey as:
Monterey is an extensible application that provides a graphical user interface for a collection of tools to simplify the creation, configuration and maintenance of Aurelia applications.
Note the word extensible in the above definition and realize that Monterey's extensibility is based on its plugins based architecture. The image below depicts several Monterey tiles (features) as well as toolbar based buttons all of which are implemented as plugins.
This whole chapter is about plugins - starting with the instructions to create a simple plugin (it has to be the Hello World plugin of course).
The next section introduces Plugin API which may not be complete yet.