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Those are the WAB (Work Area Bar), and are actually how one limits the Share/Publish Duration of the Timeline. Note: not every possible format will allow one to just Share/Publish the Timeline w/ the WAB, but the vast majority will.
This article goes into more detail on the WAB: http://forums.adobe.com/message/3961685#3961685
Good luck, and you were right where you needed to be. Just make sure to choose Work Area in the Share/Publish settings for the format that you choose.
You now have all of the pieces to the puzzle!
Do take a look at that article (I updated some images for PrE 11, as the "look" has changed, thanks to your post), as it gives you a few more uses for the WAB.
Also, I cannot recall the few formats, that do not have that Share Work Area checkbox, and, it could be that by now, Adobe has found a way to add that to every possible format. If I run across any, I will add them to the article, so that people do not think that they are missing something.