How can and can't it be used ?
This feature allows you to combine several related tags into a logical grouping. So, for instance, you might combine the tags "hitter", "pitcher", and "fielder" into a bundle and call it "Baseball".
It doesn't change anything about the existing tags, but does allow you to create another level of heirarchy. When looking at your bookmarks, del.icio.us will show this bundle and all the tags grouped under it as a separate section.
The experimental tag bundles are a great way to simulate folders in del.icio.us. To take things a step further, You can keeps all of your tags lowercase, but capitalizes the first letter of the tag bundles, making it even easier to keep track of and organize your bookmarks in bundles.
I was asked in the comments: how the grouping works? If I search for pitcher will I get all of the links tagged Baseball?
My answer: No, it won't be shown in search (or the search filter). It is simply a way to bundle all your tags in the right navigation bar. I played with it (I think two years ago). never found it to be worth the time in retrospect (because eventually, you just search all your bookmarks) I don't think there is more to know :)