Yes, believe it or not, there are still a few low level item types on which Yate does not directly allow at least some form of editing. There are also perfectly valid frames that can be added by applications that are not in the specification. iTunes has done this for a few of the sort frames. When Yate finds a frame that it does not handle, it is added to a list of unhandled frames.
All manipulation of these items is done via the context menu and is described here.
The Length field displays the size in bytes of the current selected unhandled item. You can save an item's data in one of two ways. The Export context menu item will save the object's data as an exact binary image to the output file. The Dump context menu item will save the data as a hexadecimal dump.