:index option to form_for

I was looking in the Rails source code, and during the call to form_for, it creates a new instance of FormBuilder and during the initialization, it checks if the options hash contains an index:

    @default_options = @options ? @options.slice(:index, :namespace) : {}

When will the options hash to form_for ever contain an index? I'm not sure why the options hash would ever contain an index (all the options hash contains up to this point in the form_for call are the :html options and :url option). If you look at line 1196 of form_helper.rb from latest rails release on github, you'll see that exact line a paste above: