webpack 這個(gè)配置有什么作用？

看vuex 的例子，有一個(gè)配置不清楚，在vuex examples server.js 中一個(gè)配置 publicPath: '/__build__/' , '/__build__/'是在那個(gè)目錄，運(yùn)行起來沒有這個(gè)目錄？
配置：

//server.js


const express = require('express')
const webpack = require('webpack')
const webpackDevMiddleware = require('webpack-dev-middleware')
const webpackHotMiddleware = require('webpack-hot-middleware')
const WebpackConfig = require('./webpack.config')

const app = express()
const compiler = webpack(WebpackConfig)

app.use(webpackDevMiddleware(compiler, {
  publicPath: '/__build__/',//這個(gè)路徑是在那？
  stats: {
    colors: true,
    chunks: false
  }
}))

app.use(webpackHotMiddleware(compiler))

app.use(express.static(__dirname))

const port = process.env.PORT || 8080
module.exports = app.listen(port, () => {
  console.log(`Server listening on http://localhost:${port}, Ctrl+C to stop`)
})

vuex的源碼：
https://github.com/vuejs/vuex...