[Zachery]

$post is for the postbit. All variables are only available for the templates that within scope.

ad_showthread_firstpost is an ad template, it also happens to be inside of the postbit area, so it should understand to an extent the post var scope. $post is the var that holds all of the data for the postbit of the user who posted.

The ad_header_logo template, isn't inside of the postbit, so it doesn't know what user posted a non existent post.

Depending on what you want to do, I assume for ad_header_logo you want to show this based off of the user who is already viewing the page? Is that correct? Then user $bbuserinfo


$post = postbit* related templates, for the user who actually posted the specific post
$thread/$threadinfo = related to the thread, in the showthread and postbit templates
$bbuserinfo = user who is currently viewing the site.
$vboption = should contain a lot of info, like the admin settings

Those are the biggest ones.