What is an "async relationship"? / `{async: true}` vs. `{async: false}`

Ember Data
1

The Ember Data API mentions the concept of “async relationships”.
E.g., DS.belongsTo(..) and DS.hasMany(..) both allow to provide a hash with key async:

App.Post = DS.Model.extend({
  comments: DS.hasMany('comment', {async: true})
});

What is the difference of async being either true or false and what consequences does it have on application design?

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