[Yellow Slider]

Template hooks - hooks that are inside templates.
Plugin hooks - hooks that are inside core php files.
In order to add something to the postbit_end template hook, you would want to create a new plugin using postbit_display_complete hook location, and add your own content to $template_hook['postbit_end'].

As for your second question, you could add an if statement to the postbit template, something like this:

<vb:if condition="$postinfo['postid'] == X">
POSTBIT CODE
</vb:if>