When do I use ‘use’ and ‘loader’ in Webpack 2 module.rules?

As the Webpack 2 migration tutorial states, the difference between both is, that if when we want an array of loaders, we have to use use, if it’s just one loader, then we have to use loader:

module: {
   rules: [
      {
        test: /\.jsx$/,
        loader: "babel-loader", // Do not use "use" here
        options: {
          // ...
        }
      },
      {
        test: /\.less$/,
        loader: "style-loader!css-loader!less-loader"
        use: [
          "style-loader",
          "css-loader",
          "less-loader"
        ]
      }
    ]
  }