The difference between these macros is that order id was in the old admin panel, and the campaign id is in the new one. Therefore, the new admin panel is suitable for campaign id, since there are a lot of old format orders in the new admin area.
When using direct link for skimmed traffic the publisher should apply ?bu= at the end of the tag (as long as this parameter is the first one he is using in this link) and then add any back URL he wants to use there.
For pop unders (when the direct link function has been activated in publisher's account) he should apply &bu= at the end of the link (in case there are already other parameters in the URLs) and then apply any back URL he wants to send unsold traffic to
