docker volume ls to see the names of
CaptainDuckDuck allows you to map ports from a container to the host. You should use this feature if you want a specific port of your apps/containers to be publicly accessible. The most common use case is when you want to connect to a database container from your local machine. Note that even if you don't set any port mapping, all ports are accessible from other containers on the same Captain cluster. Therefore, you should only use this option if you want the port to be publicly accessible. Make sure to have the port open, see firewall settings.
Note that you only need port mapping if your database is being accessed from an external machine. For example, if you want your NodeJS app to access your MongoDB database, and you do not need to access your MongoDB from your laptop, you don't need Port Mapping. Instead, you can use the fully qualified name for the MongoDB instance which is srv-captain--mongodb-app-name (replace mongodb-app-name with the app name you used).