You can place external plugins in this directory and they'll be included in the classpath at startup. You can then refer to them by its full type name in application.conf. For example:
{type: "commandcenter.strokeorder.StrokeOrderCommand$"}
Note: The '$' is needed as it's referring to the StrokeOrderCommand object, not class.
Besides the installation and configuration step above, there is nothing different between external Commands and built-in Commands.
You can refer to the extras module for example external plugins, such as StrokeOrderCommand.
Currently external plugins don't work with Graal native-image due to limitations regarding reflection.