What is the "only" argument of pm2-docker for?

pm2-docker is supposed to run inside docker containers, so...according to http://pm2.keymetrics.io/docs/usage/docker-pm2-nodejs/:

To split each processes in his own Docker, you can use the –only [app-name] option:

CMD ["pm2-docker", "process.yml", "--only", "APP"]

What is this --only option for?

"Split each process in his own Docker". ? His own Docker what? Container?

So pm2-docker runs inside a container and spawns containers inside of it?

via Félix Sanz