The A indicates that it is special use cable and suitable for repeated flexible use. SOW with the A indicator is called reel and pendant cable.
It is used for cranes and lifts where the cable has to be constantly on the move. I had to use some in a manlift I fixed years ago.
In radio, the A means flexible. RG-58U has a solid center whereas RG-58 UA has fine stranded wire for the center conductor.
I am wondering where this comes from. If you look here:
http://www.ul.com/global/documents/...ctrical/newsletters/W&CMG_April2007_Final.pdf
you not find a reference to -A