How does SystemJS use the configuration under "packages"?

In systemjs.config.js, we have the packages objects which provides configurations for paths in our application.

For example, here we tell SystemJS that the default extension for the path node_modules/moment is js:

packages: {
  'node_modules/moment': {
    defaultExtension: 'js'
  }
}

My question is - when does SystemJS uses this configuration? When does it actually load the file?