AFD = Active Format Description. A flag contained in either a file's metadata or VANC that tells a receiver/IRD/set-top-box how to display an image. For example, an HD file/stream may contain an AFD flag for 16:9 Letterbox, so if the file/stream is downconverted the receiver/IRD/set-top box will display the SD version letterboxed, as opposed to center cut, anamorphic, etc.
I know there are a lot of hardware items that can insert AFDs (AJA FS1, FS2, etc) and dedicated transcoders (FlipFactory, Carbon Coder, Elemental) but it would seem like this is something an editor could do as well. Maybe not, but I'd like to know for certain...