AFDs are not part of the core MPEG standard; they were originally developed within the Digital TV Group in the UK and submitted to DVB as an extension, which has subsequently also been adopted by ATSC (with some changes).
This is because the MPEG signalling can only change with a new Group of Pictures in the sequence, which is typically around every 12 frames or half a second - this was not considered accurate enough for some broadcasters who were initially switching frequently between 4:3 and 16:9.
Other widescreen 16:9 content (like sports coverage) may be broadcast with AFD 15, indicating that it is safe to display only the central 4:3 region.
More fully featured receivers also support the "safe area" information, and will use this to optimise the display for the shape of the viewer's screen.
Display in the compromise 14:9 letterbox format was not supported by initial British receivers, which limited the value of the AFD flags - this ratio is especially useful when watching widescreen material on smaller 4:3 sets.
The most common forms of down-conversion are letterbox or center-cut (cropping off the left and right sides of the 16:9 image to fit into the 4:3 raster).
Some US broadcasters transmit AFD with their HD DTV signals in order to maintain control over how SD viewers will receive their programming.
With AFD included in these signals, cable and satellite providers are able to dynamically control whether HD content is to be either letterbox or center-cut for their SD viewers.