With auto-extend groups, items can be grouped together so that when any one of those items auto-extends, every item in the group will extend together. All other items in the catalog close as expected.

Within an item's settings, you have the ability to assign items to the same group (i.e. assigning all items the same letter). Any item from this group will extend together separately from the rest of the auction. 

Bidders can see what items are grouped together by the auto-extend group icon located in the bottom right of the item's photo on the catalog view of the auction.

You can see in the screenshot above