Usage of the "Letterbox: Top & Bottom Cropped" flag?

In PgcEdit's Domain Stream Attributes dialog, there is a Letterbox: Top & Bottom Cropped flag. I have added this flag because it is present in the IFOs, but honestly I don't understand his usage. I have never seen this flag set on a commecrial DVD. If I set it and try the DVD on my standalone, I can't notice any difference. I suppose this flag is useful only when the letterbox mode is active. So, should I disable it if the user removes the letterbox mode? Any advice will be appreciated. Thanks.

It's for 4:3 video that has already been letterboxed. A lot of movies were telecined and letterboxed for broadcast, this flag let the studios make a cheap DVD by using the D1 tapes as source.

Thanks. So, if I understand correctly, this flag should be used only on a 4:3 video with black borders. It tells the DVD player to zoom in the video to simulate a 16:9 AR, but not anamorphic. Right? Maybe my old Sony lacks this function, because when used in conjunction with a 4:3 video, the output is still in 4:3, with black borders. Or maybe the flag tells only the player to output unmodified 4:3 video, but with the 16:9 signal, and it is the responsability of the 16:9 TV to zoom in the image? In this case, it doesn't work because I use the RGB output? Question. In PgcEdit, should I prohibit the usage of this flag when the 16:9 bit is set?

Great! As usual, you know every details of every question! Strange thing: I have a 16:9 TV connected with a SCRAT connector, but I have not noticed the change when I've tried this bit in various configurations. I will do another test. Thanks again.