关于use ... as MustVerifyEmailContract;等的疑惑

<?php

namespace App\Models;

use Illuminate\Contracts\Auth\MustVerifyEmail as MustVerifyEmailContract;
use Illuminate\Foundation\Auth\User as Authenticatable;
use Illuminate\Notifications\Notifiable;
use Illuminate\Auth\MustVerifyEmail as MustVerifyEmailTrait;

class User extends Authenticatable implements MustVerifyEmailContract
{
    use Notifiable, MustVerifyEmailTrait;

为什么要use…as…?
use Notifiable, MustVerifyEmailTrait;这里为什么也要use?
文中的代码解析是加载使用 MustVerifyEmail trait，这里trait是什么意思，指什么？