what is the difference between MASTER / SUB REQUEST in Symfony2?

Question

so im reading the internals documentation for symfony2 http://symfony.com/doc/current/book/internals.html and i dont understand this section http://symfony.com/doc/current/b

Answer 1

The master request is the one that comes from the original user; the subrequest is the one that you do internally — either with the forward() method of HttpKernel — or by the forward() helper of the framework's Controller class — or {% render ... %} in Twig.

Answer 2

The master request is the one, that is triggered by the browser and the sub requests are requests from within the application. For example a template can render another action

<div id="sidebar">
    {% render "AcmeArticleBundle:Article:recentArticles" with {'max': 3} %}
</div>

(Example taken from the manual)

This will lead to a sub request.