Interesting. If it a standard mobi tag is it something that can be added to Calibre? Even mobi books I download from Feedbooks do not support this feature (though it looks like Hadrien is working on it).
It is a very nice feature. With books I can use 5-way stick to go backwards or forwards by chapter. In periodicals and blogs I can go through the articles quickly. I use it much more for periodicals and blogs than books. In the progress bar at bottom you can see little dots where the chapters or articles are. When you click to the right the progress bar fills to the next dot. It has worked for all books, periodicals, and blogs purchased through Amazon.