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Just in my opition, not very clear of it, hope there will be more authoritative answer.
1.When vue-router instance initialization finished.(Like when page refreshed, vue-router need create new instance.)
2.async hooks means routes that specific a async component.(When pack with webpack , it will split code into a chunk.)
{
path: '/payment',
component: () => import('@/view/payment'),
},
Source:stackexchange.com