Understanding HTML Template file's access to Context, Passed-in Arguments, tags ?

[tian-yan]

In the default template file form.html for django-comments-xtd package,

1. context is accessible; for example {{ field }}, {{ form.comment}}, {{form}} .
   Q1: Where is the context defined and why we use {{ field }} {{ form }} instead of {{ object }} by default ?
2. passed-in arguments to view function, such as request.user.is_authenticated.
   Q2: is the instance of HttpRequest (named request) part of the template context ? or template file has access to passed-in arguments natively ?